Appendix F. Well-Known IP Protocol Numbers

A higher-layer protocol is identified with an 8-bit field within an IPv4 packet called Protocol. Figure F-1 shows the IPv4 header format, with the Protocol field shaded. Figure F-2 shows the IPv6 header format, where the protocol number is stored in the shaded Next Header field.

Figure F-1. IPv4 Header Format Showing the Protocol Field

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Figure F-2. IPv6 Base Header Format Showing the Next Header Field

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Well-known or assigned IP protocols are registered with the Internet Assigned Numbers Authority (IANA). ...

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