List of IP protocol numbers
The field protocol (protocol ) in the IPv4 header indicates to which protocol (also called "follow protocol " ) are transported in the relevant IPv4 packet payload. The field is 8 bits wide and is interpreted as an unsigned integer. Therefore, it can hold values from 0 to 255 (decimal). The values were defined in RFC 1700. Since 2002, an online database for protocol numbers with IANA was defined as up to date by RFC 3232. However, in networks are also present protocols that are not registered ( yet) with the IANA.
In the IPv6 header, there is this field too, but it says Next Header. The allowed values are the same as in the protocol field in IPv4.