A-1. IP Protocol Numbers
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.
Figure A-1. IPv4 Header Format Showing the Protocol Field
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.
| 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 ... |
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