discover a P-CSCF.
The UE can freely select method I, II, or III for P-CSCF discovery. If DHCP is used, the following procedures apply:
Upon establishing an IP-CAN bearer, the UE may use the Dynamic Host Configuration Protocol (DHCP) specified in RFC 2131
or Dynamic Host Configuration Protocol for IPv6 (DHCPv6) specified in RFC 3315
to discover the P-CSCF.
Prior to accessing the DHCP server, the UE will have obtained an IP address via means other than DHCP and DHCPv6, see 3GPP2 X.S0011-E 
If the UE uses DHCP for P-CSCF discovery and the UE is unaware of the address of the DHCP server, the UE shall send the DHCPINFORM using the limited broadcast IP address (i.e., 255.255.255.255) and UDP port 67. If the UE knows the IP address of the DHCP server, the UE shall send the DHCPINFORM to the DHCP server's unicast IP address and UDP port 67. The DHCP server shall send the DHCPACK on the IP address specified in the Client IP Address field of the DHCPINFORM. The DHCP server may include, in the DHCPACK, the SIP Server DHCP Option specified in RFC 3361
, which carries either a list of IPv4 address(es) of the P-CSCF(s) or a list of DNS fully qualified domain name(s) that can be mapped to one or more P-CSCF(s). If the UE uses DHCPv6 for P-CSCF discovery and the UE is unaware of the address of the DHCP Server, the UE shall send an Information Request using the IPv6 multicast address FF02::1:2 and the UDP port 547. If the UE knows the IP address of the DHCPv6 server, the UE shall send the Information Request message to the DHCPv6 server's IP address and UDP port 547. In the Information Request, the UE may request either one or both the SIP Servers Domain Name List option and the SIP Servers IPv6 Address List option specified in RFC 3319
. The DHCP server shall send the Reply to the IP address specified in the Information Request. The DHCP server may include in the Reply either one or both the SIP Servers Domain Name List option and the SIP Servers IPv6 Address List option, as requested by the UE.
In case several P-CSCF's IP addresses or domain names are provided to the UE, the UE shall perform P-CSCF selection according to RFC 3361
or RFC 3319
. The UE shall perform the procedure for the resolution of domain name according to RFC 3263