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
Figure F-2. IPv6 Base Header Format Showing the Next Header Field
Well-known or assigned IP protocols are registered with the Internet Assigned Numbers Authority (IANA). ...