A higher-layer protocol is identified with an 8-bit field called Protocol in an IPv4 packet. Figure A-1 shows the IPv4 packet header format, with the Protocol field shaded.
Cisco firewalls have keywords that can be used to specify certain IP protocols in access lists. These keywords are shown along with the IP protocol numbers in Table A-1.
Table A-2. Cisco Firewall Keywords for IP Protocols
Protocol Number | Firewall Keyword | IP Protocol Description |
|---|
1 | icmp | Internet Control Message Protocol (ICMP) |
2 | igmp | Internet Group Management Protocol (IGMP) |
4 | ipinip | IP-in-IP encapsulation |
6 | tcp | Transmission Control Protocol (TCP) |
9 | igrp | Interior Gateway Routing Protocol (IGRP) |
17 | udp | User Datagram Protocol (UDP) |
47 | gre or pptp | General Routing Encapsulation (GRE) |
50 | esp or ipsec | Encapsulating Security Payload (ESP) |
51 | ah | Authentication Header (AH) |
58 | icmp6 | ICMP for IPv6 |
88 | eigrp | Enhanced Interior Gateway Routing Protocol (EIGRP) |
89 | ospf | Open Shortest Path First (OSPF) |
94 | nos | Network Operating System (Novell NetWare) |
103 | pim | Protocol-Independent Multicast (PIM) |
108 | pcp | Payload Compression Protocol |
109 | snp | Sitara Networks Protocol |