Network Working Group V. Fuller
Request for Comments: 1519 BARRNet
Obsoletes: 1338 T. Li
Category: Standards Track cisco
September 1993 Classless Inter-Domain Routing (CIDR):
an Address Assignment and Aggregation Strategy
Status of this Memo
This RFC specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
This memo discusses strategies for address assignment of the existing
IP address space with a view to conserve the address space and stem
the explosive growth of routing tables in default-route-free routers.
Table of Contents
Acknowledgements ................................................. 21. Problem, Goal, and Motivation ................................ 22. CIDR address allocation ...................................... 32.1 Aggregation and its limitations ............................. 32.2 Distributed network number allocation ....................... 53. Cost-benefit analysis ........................................ 63.1 Present allocation figures .................................. 73.2 Historic growth rates ....................................... 83.3 Detailed analysis ........................................... 83.3.1 Benefits of new addressing plan ........................... 93.3.2 Growth rate projections ................................... 94. Changes to inter-domain routing protocols and practices ...... 114.1 Protocol-independent semantic changes ....................... 114.2 Rules for route advertisement ............................... 114.3 How the rules work .......................................... 134.4 Responsibility for and configuration of aggregation ......... 144.5 Intra-domain protocol considerations ........................ 155. Example of new allocation and routing ........................ 15
5.1 Address allocation .......................................... 155.2 Routing advertisements ...................................... 176. Extending CIDR to class A addresses .......................... 187. Domain Naming Service considerations ......................... 207.1 Procedural changes for class-C "supernets" ................... 207.2 Procedural changes for class-A subnetting .................... 218. Transitioning to a long term solution ........................ 229. Conclusions .................................................. 2210. Recommendations ............................................. 2211. References .................................................. 2312. Security Considerations ..................................... 2313. Authors' Addresses .......................................... 24Acknowledgements
The authors wish to express their appreciation to the members of the
ROAD group with whom many of the ideas contained in this document
were inspired and developed.
1. Problem, Goal, and Motivation
As the Internet has evolved and grown over in recent years, it has
become evident that it is soon to face several serious scaling
problems. These include:
1. Exhaustion of the class B network address space. One
fundamental cause of this problem is the lack of a network
class of a size which is appropriate for mid-sized
organization; class C, with a maximum of 254 host
addresses, is too small, while class B, which allows up to
65534 addresses, is too large for most organizations.
2. Growth of routing tables in Internet routers beyond the
ability of current software, hardware, and people to
3. Eventual exhaustion of the 32-bit IP address space.
It has become clear that the first two of these problems are likely
to become critical within the next one to three years. This memo
attempts to deal with these problems by proposing a mechanism to slow
the growth of the routing table and the need for allocating new IP
network numbers. It does not attempt to solve the third problem,
which is of a more long-term nature, but instead endeavors to ease
enough of the short to mid-term difficulties to allow the Internet to
continue to function efficiently while progress is made on a longer-
The proposed solution is to topologically allocate future IP address
assignment, by allocating segments of the IP address space to the
transit routing domains.
This plan for allocating IP addresses should be undertaken as soon as
possible. We believe that this will suffice as a short term
strategy, to fill the gap between now and the time when a viable long
term plan can be put into place and deployed effectively. This plan
should be viable for at least three (3) years, after which time,
deployment of a suitable long term solution is expected to occur.
This plan is primarily directed at the first two problems listed
above. We believe that the judicious use of variable-length
subnetting techniques should help defer the onset of the last problem
problem, the exhaustion of the 32-bit address space. Note also that
improved tools for performing address allocation in a "supernetted"
and variably-subnetted world would greatly help the user community in
accepting these sometimes confusing techniques. Efforts to create
some simple tools for this purpose should be encouraged by the
Note that this plan neither requires nor assumes that already
assigned addresses will be reassigned, though if doing so were
possible, it would further reduce routing table sizes. It is assumed
that routing technology will be capable of dealing with the current
routing table size and with some reasonably small rate of growth.
The emphasis of this plan is on significantly slowing the rate of
Note that this plan does not require domains to renumber if they
change their attached transit routing domain. Domains are encouraged
to renumber so that their individual address allocations do not need
to be advertised.
This plan will not affect the deployment of any specific long term
plan, and therefore, this document will not discuss any long term
plans for routing and address architectures.
2. CIDR address allocation
There are two basic components of this addressing and routing plan:
one, to distribute the allocation of Internet address space and two,
to provide a mechanism for the aggregation of routing information.
2.1 Aggregation and its limitations
One major goal of this addressing plan is to allocate Internet
address space in such a manner as to allow aggregation of routing
information along topological lines. For simple, single-homed
clients, the allocation of their address space out of a transit
routing domain's space will accomplish this automatically - rather
than advertise a separate route for each such client, the transit
domain may advertise a single aggregate route which describes all of
the destinations connected to it. Unfortunately, not all sites are
singly-connected to the network, so some loss of ability to aggregate
is realized for the non-trivial cases.
There are two situations that cause a loss of aggregation efficiency.
o Organizations which are multi-homed. Because multi-homed
organizations must be advertised into the system by each of
their service providers, it is often not feasible to
aggregate their routing information into the address space
any one of those providers. Note that they still may receive
their address allocation out of a transit domain's address
space (which has other advantages), but their routing
information must still be explicitly advertised by most of
their service providers (the exception being that if the
site's allocation comes out of its least-preferable service
provider, then that service provider need not advertise the
explicit route - longest-match will insure that its
aggregated route is used to get to the site on a backup
basis). For this reason, the routing cost for these
organizations will typically be about the same as it is
o Organizations which change service provider but do not
renumber. This has the effect of "punching a hole" in the
aggregation of the original service provider's advertisement.
This plan will handle the situation by requiring the newer
service provider to advertise a specific advertisement for
the new client, which is preferred by virtue of being the
longest match. To maintain efficiency of aggregation, it is
recommended that organizations which do change service
providers plan to eventually migrate their address
assignments from the old provider's space to that of the new
provider. To this end, it is recommended that mechanisms to
facilitate such migration, including improved protocols and
procedures for dynamic host address assignment, be developed.
Note that some aggregation efficiency gain can still be had for
multi-homed sites (and, in general, for any site composed of
multiple, logical IP network numbers) - by allocating a contiguous
power-of-two block of network numbers to the client (as opposed to
multiple, independent network numbers) the client's routing
information may be aggregated into a single (net, mask) pair. Also,
since the routing cost associated with assigning a multi-homed site
out of a service provider's address space is no greater than the
current method of a random allocation by a central authority, it
makes sense to allocate all address space out of blocks assigned to
It is also worthwhile to mention that since aggregation may occur at
multiple levels in the system, it may still be possible to aggregate
these anomalous routes at higher levels of whatever hierarchy may be
present. For example, if a site is multi-homed to two NSFNET regional
networks both of whom obtain their address space from the NSFNET,
then aggregation by the NSFNET of routes from the regionals will
include all routes to the multi-homed site.
Finally, it should also be noted that deployment of the new
addressing plan described in this document may (and should) begin
almost immediately but effective use of the plan to aggregate routing
information will require changes to some Inter-Domain routing
protocols. Likewise, deploying classless Inter-Domain protocols
without deployment of the new address plan will not allow useful
aggregation to occur (in other words, the addressing plan and routing
protocol changes are both required for supernetting, and its
resulting reduction in table growth, to be effective.) Note,
however, that during the period of time between deployment of the
addressing plan and deployment of the new protocols, the size of
routing tables may temporarily grow very rapidly. This must be
considered when planning the deployment of the two plans.
Note: in the discussion and examples which follow, the network and
mask notation is used to represent routing destinations. This is used
for illustration only and does not require that routing protocols use
this representation in their updates.
2.2 Distributed allocation of address space
The basic idea of the plan is to allocate one or more blocks of Class
C network numbers to each network service provider. Organizations
using the network service provider for Internet connectivity are
allocated bitmask-oriented subsets of the provider's address space as
It is also worthwhile to mention that once inter-domain protocols
which support classless network destinations are widely deployed, the
rules described by this plan generalize to permit arbitrary
super/subnetting of the remaining class A and class B address space
(the assumption being that classless inter-domain protocols will
either allow for non-contiguous subnets to exist in the system or
that all components of a sub-allocated class A/B will be contained
within a single routing domain). This will allow this plan to
continue to be used in the event that the class C space is exhausted
before implementation of a long-term solution is deployed. This
alternative is discussed further below in section 6.
Hierarchical sub-allocation of addresses in this manner implies that
clients with addresses allocated out of a given service provider are,
for routing purposes, part of that service provider and will be
routed via its infrastructure. This implies that routing information
about multi-homed organizations, i.e., organizations connected to
more than one network service provider, will still need to be known
by higher levels in the hierarchy.
The advantages of hierarchical assignment in this fashion are
a) It is expected to be easier for a relatively small number of
service providers to obtain addresses from the central
authority, rather than a much larger, and monotonically
increasing, number of individual clients. This is not to be
considered as a loss of part of the service providers' address
b) Given the current growth of the Internet, a scalable and
delegatable method of future allocation of network numbers has
to be achieved.
For these reasons, and in the interest of providing a consistent
procedure for obtaining Internet addresses, it is recommended that
most, if not all, network numbers be distributed through service
providers. These issues are discussed in much greater length in .
3. Cost-benefit analysis
This new method of assigning address through service providers can be
put into effect immediately and will, from the start, have the
benefit of distributing the currently centralized process of
assigning new addresses. Unfortunately, before the benefit of
reducing the size of globally-known routing destinations can be
achieved, it will be necessary to deploy an Inter-Domain routing
protocol capable of handling arbitrary network and mask pairs. Only
then will it be possible to aggregate individual class C networks
into larger blocks represented by single routing table entries.
This means that upon introduction, the new addressing allocation plan
will not in and of itself help solve the routing table size problem.
Once the new Inter-Domain routing protocol is deployed, however, an
immediate drop in the number of destinations which clients of the new
protocol must carry will occur. A detailed analysis of the magnitude
of this expected drop and the permanent reduction in rate of growth
is given in the next section.
In should also be noted that the present method of flat address
allocations imposes a large bureaucratic cost on the central address
allocation authority. For scaling reasons unrelated to address space
exhaustion or routing table overflow, this should be changed. Using
the mechanism proposed in this paper will have the fortunate side
effect of distributing the address allocation procedure, greatly
reducing the load on the central authority.
3.1 Present Allocation Figures
An informal analysis of "network-contacts.txt" (available from the
DDN NIC) indicates that as of 2/25/92, 46 of 126 class A network
numbers have been allocated (leaving 81) and 5467 of 16382 class B
numbers have been allocated, leaving 10915. Assuming that recent
trends continue, the number of allocated class B's will continue to
double approximately once a year. At this rate of growth, all class
B's will be exhausted within about 15 months. As of 1/13/93, 52
class A network numbers have been allocated and 7133 class B's have
been allocated. We suggest that the change in the class B allocation
rate is due to the initial deployment of this address allocation
3.2 Historic growth rates
MM/YY ROUTES MM/YY ROUTES
Dec-92 8561 Sep-90 1988
Nov-92 7854 Aug-90 1894
Oct-92 7354 Jul-90 1727
Sep-92 6640 Jun-90 1639
Aug-92 6385 May-90 1580
Jul-92 6031 Apr-90 1525
Jun-92 5739 Mar-90 1038
May-92 5515 Feb-90 997
Apr-92 5291 Jan-90 927
Mar-92 4976 Dec-89 897
Feb-92 4740 Nov-89 837
Jan-92 4526 Oct-89 809
Dec-91 4305 Sep-89 745
Nov-91 3751 Aug-89 650
Oct-91 3556 Jul-89 603
Sep-91 3389 Jun-89 564
Aug-91 3258 May-89 516
Jul-91 3086 Apr-89 467
Jun-91 2982 Mar-89 410
May-91 2763 Feb-89 384
Apr-91 2622 Jan-89 346
Mar-91 2501 Dec-88 334
Feb-91 2417 Nov-88 313
Jan-91 2338 Oct-88 291
Dec-90 2190 Sep-88 244
Nov-90 2125 Aug-88 217
Oct-90 2063 Jul-88 173
Table I : Growth in routing table size, total numbers
Source for the routing table size data is MERIT
3.3 Detailed Analysis
There is a small technical cost and minimal administrative cost
associated with deployment of the new address assignment plan. The
administrative cost is basically that of convincing the NIC, the
IANA, and the network service providers to agree to this plan, which
is not expected to be too difficult. In addition, administrative
cost for the central numbering authorities (the NIC and the IANA)
will be greatly decreased by the deployment of this plan. To take
advantage of aggregation of routing information, however, it is
necessary that the capability to represent routes as arbitrary
network and mask fields (as opposed to the current class A/B/C
distinction) be added to the common Internet inter-domain routing
protocol(s). Thus, the technical cost is in the implementation of
classless interdomain routing protocols.
3.3.1 Benefits of the new addressing plan
There are two benefits to be had by deploying this plan:
o The current problem with depletion of the available class B
address space can be ameliorated by assigning more-
appropriately sized blocks of class C's to mid-sized
organizations (in the 200-4000 host range).
o When the improved inter-domain routing protocol is deployed,
an immediate decrease in the number routing table entries
should occur, followed by a significant reduction in the rate
growth of routing table size (for default-free routers).
3.3.2 Growth rate projections
As of Jan '92, a default-free routing table (for example, the routing
tables maintained by the routers in the NSFNET backbone) contained
approximately 4700 entries. This number reflects the current size of
the NSFNET routing database. Historic data shows that this number, on
average, has doubled every 10 months between 1988 and 1991. Assuming
that this growth rate is going to persist in the foreseeable future
(and there is no reason to assume otherwise), we expect the number of
entries in a default-free routing table to grow to approximately
30000 in two years time. In the following analysis, we assume that
the growth of the Internet has been, and will continue to be,
It should be stressed that these projections do not consider that the
current shortage of class B network numbers may increase the number
of instances where many class C's are used rather than a class B.
Using an assumption that new organizations which formerly obtained
class B's will now obtain somewhere between 4 and 16 class C's, the
rate of routing table growth can conservatively be expected to at
least double and probably quadruple. This means the number of entries
in a default-free routing table may well exceed 10,000 entries within
six months and 20,000 entries in less than a year.
As of Dec '92, the routing table contains 8500 routes. The original
growth curves would predict over 9400 routes. At this time, it is
not clear if this would indicate a significant change in the rate of
Under the proposed plan, growth of the routing table in a default-
free router is greatly reduced since most new address assignment will
come from one of the large blocks allocated to the service providers.
For the sake of this analysis, we assume prompt implementation of
this proposal and deployment of the revised routing protocols. We
make the initial assumption that any initial block given to a
provider is sufficient to satisfy its needs for two years.
Since under this plan, multi-homed networks must continue to be
explicitly advertised throughout the system (according to Rule #1
described in section 4.2), the number multi-homed routes is expected
to be the dominant factor in future growth of routing table size,
once the supernetting plan is applied.
Presently, it is estimated that there are fewer than 100 multi-homed
organizations connected to the Internet. Each such organization's
network is comprised of one or more network numbers. In many cases
(and in all future cases under this plan), the network numbers used
by an organization are consecutive, meaning that aggregation of those
networks during route advertisement may be possible. This means that
the number of routes advertised within the Internet for multi-homed
networks may be approximated as the total number of multi-homed
organizations. Assuming that the number of multi-homed organization
will double every year (which may be a over-estimation, given that
every connection costs money), the number of routes for multi-homed
networks would be expected to grow to approximately 800 in three
If we further assume that there are approximately 100 service
providers, then each service provider will also need to advertise its
block of addresses. However, due to aggregation, these
advertisements will be reduced to only 100 additional routes. We
assume that after the initial two years, new service providers
combined with additional requests from existing providers will
require an additional 50 routes per year. Thus, the total is 4700 +
800 + 150 = 5650. This represents an annual growth rate of
approximately 6%. This is in clear contrast to the current annual
growth of 130%. This analysis also assumes an immediate deployment
of this plan with full compliance. Note that this analysis assumes
only a single level of route aggregation in the current Internet -
intelligent address allocation should significantly improve this.
Clearly, this is not a very conservative assumption in the Internet
environment nor can 100% adoption of this proposal be expected.
Still, with only a 90% participation in this proposal by service
providers, at the end of the target three years, global routing table
size will be "only" 4700 + 800 + 145 + 7500 = 13145 routes -- without
any action, the routing table will grow to approximately 75000 routes
during that time period.
4. Changes to inter-domain routing protocols and practices
In order to support supernetting efficiently, it is clear that some
changes will need to be made to both routing protocols themselves and
to the way in which routing information is interpreted. In the case
of "new" inter-domain protocols, the actual protocol syntax changes
should be relatively minor. This mechanism will not work with older
inter-domain protocols such as EGP2; the only ways to interoperate
with old systems using such protocols are either to use existing
mechanisms for providing "default" routes or b) require that new
routers talking to old routers "explode" supernet information into
individual network numbers. Since the first of these is trivial
while the latter is cumbersome (at best -- consider the memory
requirements it imposes on the receiver of the exploded information),
it is recommended that the first approach be used -- that older
systems to continue to the mechanisms they currently employ for
Note that a basic assumption of this plan is that those organizations
which need to import "supernet" information into their routing
systems must run IGPs (such as OSPF ) which support classless
routes. Systems running older IGPs may still advertise and receive
"supernet" information, but they will not be able to propagate such
information through their routing domains.
4.1 Protocol-independent semantic changes
There are two fundamental changes which must be applied to Inter-
Domain routing protocols in order for this plan to work. First, the
concept of network "class" needs to be deprecated - this plan assumes
that routing destinations are represented by network and mask pairs
and that routing is done on a longest-match basis (i.e., for a given
destination which matches multiple network+mask pairs, the match with
the longest mask is used). Second, current inter-domain protocols
generally do not support the concept of route aggregation, so the new
semantics need to be implemented in a new set of inter-domain
protocols. In particular, when doing aggregation, dealing with
multi-homed sites or destinations which change service providers is
difficult. Fortunately, it is possible to define several fairly
simple rules for dealing with such cases.
4.2. Rules for route advertisement
1. Routing to all destinations must be done on a longest-match
basis only. This implies that destinations which are multi-
homed relative to a routing domain must always be explicitly
announced into that routing domain - they cannot be summarized
(this makes intuitive sense - if a network is multi-homed, all
of its paths into a routing domain which is "higher" in the
hierarchy of networks must be known to the "higher" network).
2. A routing domain which performs summarization of multiple
routes must discard packets which match the summarization but
do not match any of the explicit routes which makes up the
summarization. This is necessary to prevent routing loops in
the presence of less-specific information (such as a default
route). Implementation note - one simple way to implement
this rule would be for the border router to maintain a "sink"
route for each of its aggregations. By the rule of longest
match, this would cause all traffic destined to components of
the aggregation which are not explicitly known to be
Note that during failures, partial routing of traffic to a site which
takes its address space from one service provider but which is
actually reachable only through another (i.e., the case of a site
which has change service providers) may occur because such traffic
will be routed along the path advertised by the aggregated route.
Rule #2 will prevent any real problem from occurring by forcing such
traffic to be discarded by the advertiser of the aggregated route,
but the output of "traceroute" and other similar tools will suggest
that a problem exists within the service provider advertising the
aggregate, which may be confusing to network operators (see the
example in section 5.2 for details). Solutions to this problem appear
to be challenging and not likely to be implementable by current
Inter-Domain protocols within the time-frame suggested by this
document. This decision may need to be revisited as Inter-Domain
An implementation following these rules should also be generalized,
so that an arbitrary network number and mask are accepted for all
routing destinations. The only outstanding constraint is that the
mask must be left contiguous. Note that the degenerate route 0.0.0.0
mask 0.0.0.0 is used as a default route and MUST be accepted by all
implementations. Further, to protect against accidental
advertisements of this route via the inter-domain protocol, this
route should never be advertised unless there is specific
configuration information indicating to do so.
Systems which process route announcements must also be able to verify
that information which they receive is correct. Thus, implementations
of this plan which filter route advertisements must also allow masks
in the filter elements. To simplify administration, it would be
useful if filter elements automatically allowed more specific network
numbers and masks to pass in filter elements given for a more general
mask. Thus, filter elements which looked like:
would look something like:
accept 220.127.116.11 255.255.0.0
accept 18.104.22.168 255.255.0.0
accept 22.214.171.124 255.255.0.0
deny 126.96.36.199 255.255.0.0
accept 188.8.131.52 255.0.0.0
This is merely making explicit the network mask which was implied by
the class A/B/C classification of network numbers.
4.3. How the rules work
Rule #1 guarantees that the routing algorithm used is consistent
across implementations and consistent with other routing protocols,
such as OSPF. Multi-homed networks are always explicitly advertised
by every service provider through which they are routed even if they
are a specific subset of one service provider's aggregate (if they
are not, they clearly must be explicitly advertised). It may seem as
if the "primary" service provider could advertise the multi-homed
site implicitly as part of its aggregate, but the assumption that
longest-match routing is always done causes this not to work.
Rule #2 guarantees that no routing loops form due to aggregation.
Consider a mid-level network which has been allocated the 2048 class
C networks starting with 184.108.40.206 (see the example in section 5 for
more on this). The mid-level advertises to a "backbone"
220.127.116.11/255.248.0.0. Assume that the "backbone", in turn, has been
allocated the block of networks 192.0.0.0/255.0.0.0. The backbone
will then advertise this aggregate route to the mid-level. Now, if
the mid-level loses internal connectivity to the network
18.104.22.168/255.255.255.0 (which is part of its aggregate), traffic
from the "backbone" to the mid-level to destination 22.214.171.124 will
follow the mid-level's advertised route. When that traffic gets to
the mid-level, however, the mid-level *must not* follow the route
192.0.0.0/255.0.0.0 it learned from the backbone, since that would
result in a routing loop. Rule #2 says that the mid-level may not
follow a less-specific route for a destination which matches one of
its own aggregated routes. Note that handling of the "default" route
(0.0.0.0/0.0.0.0) is a special case of this rule - a network must not
follow the default to destinations which are part of one of it's
4.4. Responsibility for and configuration of aggregation
The domain which has been allocated a range of addresses has the sole
authority for aggregation of its address space. In the usual case,
the AS will install manual configuration commands in its border
routers to aggregate some portion of its address space. An domain
can also delegate aggregation authority to another domain. In this
case, aggregation is done in the other domain by one of its border
When an inter-domain border router performs route aggregation, it
needs to know the range of the block of IP addresses to be
aggregated. The basic principle is that it should aggregate as much
as possible but not to aggregate those routes which cannot be treated
as part of a single unit due to multi-homing, policy, or other
One mechanism is to do aggregation solely based on dynamically
learned routing information. This has the danger of not specifying a
precise enough range since when a route is not present, it is not
always possible to distinguish whether it is temporarily unreachable
or that it does not belong in the aggregate. Purely dynamic routing
also does not allow the flexibility of defining what to aggregate
within a range. The other mechanism is to do all aggregation based on
ranges of blocks of IP addresses preconfigured in the router. It is
recommended that preconfiguration be used, since it more flexible and
allows precise specification of the range of destinations to
Preconfiguration does require some manually-maintained configuration
information, but not excessively more so than what router
administrators already maintain today. As an addition to the amount
of information that must be typed in and maintained by a human,
preconfiguration is just a line or two defining the range of the
block of IP addresses to aggregate. In terms of gathering the
information, if the advertising router is doing the aggregation, its
administrator knows the information because the aggregation ranges
are assigned to its domain. If the receiving domain has been granted
the authority to and task of performing aggregation, the information
would be known as part of the agreement to delegate aggregation.
Given that it is common practice that a network administrator learns
from its neighbor which routes it should be willing to accept,
preconfiguration of aggregation information does not introduce
additional administrative overhead.
Implementation note: aggregates which encompass the class D address
space (multicast addresses) are currently not well understood. At
present, it appears that the optimal strategy is to consider
aggregates to never encompass class D space, even if they do so
4.5 Intra-domain protocol considerations
While no changes need be made to internal routing protocols to
support the advertisement of aggregated routing information between
autonomous systems, it is often the case that external routing
information is propagated within interior protocols for policy
reasons or to aid in the propagation of information through a transit
network. At the point when aggregated routing information starts to
appear in the new exterior protocols, this practice of importing
external information will have to be modified. A transit network
which imports external information will have to do one of:
a) use an interior protocol which supports aggregated routing
b) find some other method of propagating external information
which does not involve flooding it through the interior
protocol (i.e., by the use of internal BGP, for example).
c) stop the importation of external information and flood a
"default" route through the internal protocol for discovery
of paths to external destinations.
For case (a), the modifications necessary to a routing protocol to
allow it to support aggregated information may not be simple. For
protocols such as OSPF and IS-IS, which represent routing information
as either a destination+mask (OSPF) or as a prefix+prefix-length
(IS-IS) changes to support aggregated information are conceptually
fairly simple; for protocols which are dependent on the class-A/B/C
nature of networks or which support only fixed-sized subnets, the
changes are of a more fundamental nature. Even in the "conceptually
simple" cases of OSPF and IS-IS, an implementation may need to be
modified to support supernets in the database or in the forwarding
5. Example of new allocation and routing
5.1 Address allocation
Consider the block of 2048 class C network numbers beginning with
126.96.36.199 (0xC0180000 and ending with 188.8.131.52 (0xC01FFF00)
allocated to a single network provider, "RA". A "supernetted" route
to this block of network numbers would be described as 184.108.40.206
with mask of 255.248.0.0 (0xFFF80000).
Assume this service provider connects six clients in the following
order (significant because it demonstrates how temporary "holes" may
form in the service provider's address space):
"C1" requiring fewer than 2048 addresses (8 class C networks)
"C2" requiring fewer than 4096 addresses (16 class C networks)
"C3" requiring fewer than 1024 addresses (4 class C networks)
"C4" requiring fewer than 1024 addresses (4 class C networks)
"C5" requiring fewer than 512 addresses (2 class C networks)
"C6" requiring fewer than 512 addresses (2 class C networks)
In all cases, the number of IP addresses "required" by each client is
assumed to allow for significant growth. The service provider
allocates its address space as follows:
C1: allocate 192.24.0 through 192.24.7. This block of networks is
described by the "supernet" route 220.127.116.11 and mask
C2: allocate 192.24.16 through 192.24.31. This block is described
by the route 18.104.22.168, mask 255.255.240.0
C3: allocate 192.24.8 through 192.24.11. This block is described
by the route 22.214.171.124, mask 255.255.252.0
C4: allocate 192.24.12 through 192.24.15. This block is described
by the route 126.96.36.199, mask 255.255.252.0
C5: allocate 192.24.32 and 192.24.33. This block is described by
the route 188.8.131.52, mask 255.255.254.0
C6: allocate 192.24.34 and 192.24.35. This block is described by
the route 184.108.40.206, mask 255.255.254.0
Note that if the network provider uses an IGP which can support
classless networks, he can (but doesn't have to) perform
"supernetting" at the point where he connects to his clients and
therefore only maintain six distinct routes for the 36 class C
network numbers. If not, explicit routes to all 36 class C networks
will have to be carried by the IGP.
To make this example more realistic, assume that C4 and C5 are
multi-homed through some other service provider, "RB". Further assume
the existence of a client "C7" which was originally connected to "RB"
but has moved to "RA". For this reason, it has a block of network
numbers which are allocated out "RB"'s block of (the next) 2048 class
C network numbers:
C7: allocate 192.32.0 through 192.32.15. This block is described
by the route 192.32.0, mask 255.255.240.0
For the multi-homed clients, we will assume that C4 is advertised as
primary via "RA" and secondary via "RB"; C5 is primary via "RB" and
secondary via "RA". To connect this mess together, we will assume
that "RA" and "RB" are connected via some common "backbone" provider
Graphically, this simple topology looks something like this:
220.127.116.11 -- 18.104.22.168 \ _ 22.214.171.124 - 126.96.36.199
188.8.131.52/255.255.248.0 \ / 184.108.40.206/255.255.240.0
\ / C7
C2 +----+ +----+
220.127.116.11 - 18.104.22.168 \| | | |
22.214.171.124/255.255.240.0 | | _ 126.96.36.199 - 188.8.131.52 _ | |
| | / 184.108.40.206/255.255.252.0 \ | |
C3 -| |/ C4 \| |
220.127.116.11 - 18.104.22.168 | RA | | RB |
22.214.171.124/255.255.252.0 | |___ 126.96.36.199 - 188.8.131.52 ___| |
/| | 184.108.40.206/255.255.254.0 | |
C6 | | C5 | |
220.127.116.11 - 18.104.22.168 | | | |
22.214.171.124/255.255.254.0 | | | |
126.96.36.199/255.255.252.0 (C4) || 188.8.131.52/255.255.252.0 (C4) ||
184.108.40.206/255.255.240.0 (C7) || 220.127.116.11/255.255.254.0 (C5) ||
18.104.22.168/255.248.0.0 (RA) || 22.214.171.124/255.248.0.0 (RB) ||
+--------------- BACKBONE PEER BB ---------------+
5.2 Routing advertisements
To follow rule #1, RA will need to advertise the block of addresses
that it was given and C7. Since C4 is multi-homed and primary
through RA, it must also be advertised. C5 is multi-homed and
primary through RB. It need not be advertised since longest match by
BB will automatically select RB as primary and the advertisement of
RA's aggregate will be used as a secondary.
Advertisements from "RA" to "BB" will be:
126.96.36.199/255.255.252.0 primary (advertises C4)
188.8.131.52/255.255.240.0 primary (advertises C7)
184.108.40.206/255.248.0.0 primary (advertises remainder of RA)
For RB, the advertisements must also include C4 and C5 as well as
it's block of addresses. Further, RB may advertise that C7 is
Advertisements from "RB" to "BB" will be:
220.127.116.11/255.255.252.0 secondary (advertises C4)
18.104.22.168/255.255.254.0 primary (advertises C5)
22.214.171.124/255.248.0.0 primary (advertises remainder of RB)
To illustrate the problem alluded to by the "note" in section 4.2,
consider what happens if RA loses connectivity to C7 (the client
which is allocated out of RB's space). In a stateful protocol, RA
will announce to BB that 126.96.36.199/255.255.240.0 has become
unreachable. Now, when BB flushes this information out of its routing
table, any future traffic sent through it for this destination will
be forwarded to RB (where it will be dropped according to Rule #2) by
virtue of RB's less specific match 188.8.131.52/255.248.0.0. While
this does not cause an operational problem (C7 is unreachable in any
case), it does create some extra traffic across "BB" (and may also
prove confusing to a network manager debugging the outage with
"traceroute"). A mechanism to cache such unreachability information
would help here, but is beyond the scope of this document (such a
mechanism is also not implementable in the near-term).
6. Extending CIDR to class A addresses
At some point, it is expected that this plan will eventually consume
all of the remaining class C address space. As of this writing, the
upper half of the class A address space has already been reserved for
future expansion. This section describes how the CIDR plan can be
used to utilize this portion of the class A space efficiently. It is
expected that this contingency would only be used if no long term
solution has become apparent by the time that the class C address
space is consumed.
Fundamentally, there are two differences between using a class A
address and a block of class C's. First, the configuration of DNS
becomes somewhat more complicated than it is without the aggregation
of class A subnets. The second difference is that the routers within
the class A address would need to support and use a classless IGP.
Maintenance of DNS with a subnetted class A is somewhat painful. As
part of the mechanism for providing reverse address lookups, DNS
maintains a "IN-ADDR.ARPA" reverse domain. This is configured by
reversing the dotted decimal network number, appending "IN-ADDR.ARPA"
and using this as a type of pseudo-domain. Individual hosts then end
up pointing back to a host name. Thus, for example, 184.108.40.206
has a DNS record "220.127.116.11.IN-ADDR.ARPA." Since the pseudo-
domains can only be delegated on a byte boundary, this becomes
painful if a stub domain receives a block of address space that does
not fall on a byte boundary. The solution in this case is to
enumerate all of the possible byte combinations involved. This is
painful, but workable. This is discussed further below.
Routing within a class A used for CIDR is also an interesting
challenge. The usual case will be that a domain will be assigned a
portion of the class A address space. The domain can either use an
IGP which allows variable length subnets or it can pick a single
subnet mask to be used throughout the domain. In the latter case,
difficulties arise because other domains have been allocated other
parts of the class A address space and may be using a different
subnet mask. If the domain is itself a transit, it may also need to
allocate some portion of its space to a client, which might also use
a different subnet mask. The client would then need routing
information about the remainder of the class A.
If the client's IGP does not support variable length subnet masks,
this could be done by advertising the remainder of the class A's
address space in appropriately sized subnets. However, unless the
client has a very large portion of the class A space, this is likely
to result in a large number of subnets (for example, a mask of
255.255.255.0 would require a total of 65535 subnets, including those
allocated to the client). For this reason, it may be preferable to
simply use an IGP that supports variable length subnet masks within
the client's domain.
Similarly, if a transit has been assigned address space from a class
A network number, it is likely that it was not assigned the entire
class A, and that other transit domains will get address space from
this class A. In this case, the transit would also have to inject
routing information about the remainder of the class A into it's IGP.
This is analogous to the situation above, with the same
complications. For this reason, we recommend that the use of a class
A for CIDR only be attempted if IGP's with variable length subnet
mask support be used throughout the class A. Note that the IGP's
need not support supernetting, as discussed above.
Note that the technique here could also apply to class B addresses.
However, the limited number of available class B addresses and their
usage for multihomed networks suggests that this address space should
only be reserved for those large single organizations that warrant
this type of address. 
7. Domain Service considerations
One aspect of Internet services which will be notably affected by a
move to either "supernetted" class-C network numbers or subdivided
class-A's will be the mechanism used for address-to-name translation:
the IN-ADDR.ARPA zone of the domain system. Because this zone is
delegated on octet boundaries only, any address allocation plan which
uses bitmask-oriented addressing will cause some degree of difficulty
for those which maintain parts of the IN-ADDR.ARPA zone.
7.1 Procedural changes for class-C "supernets"
At the present time, parts of the IN-ADDR.ARPA zone are delegated
only on network boundaries which happen to fall on octet boundaries.
To aid in the use of blocks of class-C networks, it is recommended
that this policy be relaxed and allow the delegation of arbitrary,
octet-oriented pieces of the IN-ADDR.ARPA zone.
As an example of this policy change, consider a hypothetical large
network provider named "BigNet" which has been allocated the 1024
class-C networks 199.0.0 through 199.3.255. Under current policies,
the root domain servers would need to have 1024 entries of the form:
0.0.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
1.0.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
255.3.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
By revising the policy as described above, this is reduced only four
0.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
1.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
2.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
3.199.IN-ADDR.ARPA. IN NS NS1.BIG.NET.
The provider would then maintain further delegations of naming
authority for each individual class-C network which it assigns,
rather than having each registered separately. Note that due to the
way the DNS is designed, it is still possible for the root
nameservers to maintain the delegation information for individual
networks for which the provider is unwilling or unable to do so. This
should greatly reduce the load on the domain servers for the "top"
levels of the IN-ADDR.ARPA domain. The example above illustrates
only the records for a single nameserver. In the normal case, there
are usually several nameservers for each domain, thus the size of the
examples will double or triple in the common cases.
7.2 Procedural changes for class-A subnetting
Should it be the case the class-A network numbers are subdivided into
blocks allocated to transit network providers, it will be similarly
necessary to relax the restriction on how IN-ADDR.ARPA naming works
for them. As an example, take a provider is allocated the 19-bit
portion of address space which matches 10.8.0.0 with mask
255.248.0.0. This represents all addresses which begin with the
prefixes 10.8, 10.9, 10.10, 10.11, 10.12, 10.13, 10.14, an 10.15 and
requires the following IN-ADDR.ARPA delegations:
8.10.IN-ADDR.ARPA. IN NS NS1.MOBY.NET.
9.10.IN-ADDR.ARPA. IN NS NS1.MOBY.NET.
15.10.IN-ADDR.ARPA. IN NS NS1.MOBY.NET.
To further illustrate how IN-ADDR.ARPA sub-delegation will work,
consider a company named "FOO" connected to this provider which has
been allocated the 14-bit piece of address space which matches
10.10.64.0 with mask 255.255.192.0. This represents all addresses in
the range 10.10.64.0 through 10.10.127.255 and will require that the
provider implement the following IN-ADDR.ARPA delegations:
64.10.10.IN-ADDR.ARPA. IN NS NS1.FOO.COM.
65.10.10.IN-ADDR.ARPA. IN NS NS1.FOO.COM.
127.10.10.IN-ADDR.ARPA. IN NS NS1.FOO.COM.
with the servers for "FOO.COM" containing the individual PTR records
for all of the addresses on each of these subnets.
8. Transitioning to a long term solution
This solution does not change the Internet routing and addressing
architectures. Hence, transitioning to a more long term solution is
not affected by the deployment of this plan.
We are all aware of the growth in routing complexity, and the rapid
increase in allocation of network numbers. Given the rate at which
this growth is being observed, we expect to run out in a few short
If the inter-domain routing protocol supports carrying network routes
with associated masks, all of the major concerns demonstrated in this
paper would be eliminated.
One of the influential factors which permits maximal exploitation of
the advantages of this plan is the number of people who agree to use
If service providers start charging networks for advertising network
numbers, this would be a very great incentive to share the address
space, and hence the associated costs of advertising routes to
The NIC should begin to hand out large blocks of class C addresses to
network service providers. Each block must fall on bit boundaries
and should be large enough to serve the provider for two years.
Further, the NIC should distribute very large blocks to continental
and national network service organizations to allow additional levels
of aggregation to take place at the major backbone networks. In
addition, the NIC should modify its procedures for the IN-ADDR.ARPA
domain to permit delegation along arbitrary octet boundaries.
Service providers will further allocate power-of-two blocks of class
C addresses from their address space to their subscribers.
All organizations, including those which are multi-homed, should
obtain address space from their provider (or one of their providers,
in the case of the multi-homed). These blocks should also fall on
bit boundaries to permit easy route aggregation.
To allow effective use of this new addressing plan to reduce
propagated routing information, appropriate IETF WGs will specify the
modifications needed to Inter-Domain routing protocols.
Implementation and deployment of these modifications should occur as
quickly as possible.
 Moy, J, "The OSPF Specification Version 2", RFC 1247, Proteon,
Inc., January 1991.
 Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM
Corp., cisco Systems, September 1993.
12. Security Considerations
Security issues are not discussed in this memo.
13. Authors' Addresses
Pine Hall 115
Stanford, CA, 94305-4122
cisco Systems, Inc.
1525 O'Brien Drive
Menlo Park, CA 94025
Jessica (Jie Yun) Yu
Merit Network, Inc.
1071 Beal Ave.
Ann Arbor, MI 48109
Internet Engineer, OARnet
1224, Kinnear Road,
Columbus, OH 43212