when looking at this list here https://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

I see IPv6 was given #41, or 0x29.

But there are many protocols after #41. Are these other protocols assigned below because they were created after IPv6?

As of today, Nov 11, 2022, it shows 144-252 as free unassigned addresses for the "protocol" field.

Will the next RFC that comes into existence snag "144" as the assigned address? Are we to assume every protocol in that list, is assigned in chronological nature?

  • 1
    You're slightly mistaken in assuming that that IPv6 has protocol number 41. Protocol 41 only applies to IPv4, and is used when transporting IPv6 packets within IPv4 packets. Other than that, where needed, IPv6 has it's own L3 protocol identifier: for example when transported in within ethernet frames, it is identified by Ethertype 0x86dd (where IPv4 has 0x0800), or 0x0057 when transported within PPP. Nov 12, 2022 at 13:46

2 Answers 2


Are the IP protocols contained in the Protocol Field in IPv4 header, chronologically assigned by date of creation?

Not necessarily, but also remember that the first RFC for IPv6 was published in 1995, nearly 30 years ago.

You often get what you pay for when using free services. What you see there for protocol number 41 is not IPv6, it is IPv6 encapsulation defined by RFC 2473, Generic Packet Tunneling in IPv6 Specification several years after the RFC for IPv6.

IANA (Internet Assigned Numbers Authority) maintains the official list of protocol numbers. See the IANA Protocol Numbers page for the list of protocol numbers and their source.


Nope, they are definitely not numbered in chronological order of creation/adoption because some of the IPv6 ones were created much later (at least about 4-6 years or so) than some of the higher numbered encryption related ones, etc.

You can safely assume they are numbered on a first come, first served basis with adjustments for 'We feel like it should be this number'-style decisions. Some numbers that may have been assigned at some point in the past will be reused if those protocols are unused or fall out of favor for long enough or are replaced with new protocols that effectively replace them.

Also, it doesn't matter at all which is assigned which number as long as they can be uniquely identified to a meaningful degree.

  • IIRC, some of the numbers were also officially assigned after they had become de-facto standards in the field. IOW, people were already using the protocol number for a specific purpose regularly, so IANA just assigned that number to that purpose officially to keep compatibility with existing systems in the wild. Nov 13, 2022 at 2:14

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