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I found that Internet Protocol (IP) version numbers are published by IANA. It refers to the following table.

https://www.iana.org/assignments/version-numbers/version-numbers.xhtml

Decimal Keyword Version Reference
0-1 Reserved Jon_Postel, RFC4928
2-3 Unassigned Jon_Postel
4 IP Internet Protocol RFC791, Jon_Postel
5 Reserved (Historic) RFC1819, SC589H
6 IPv6 Internet Protocol version 6 RFC8200
7 Reserved (Historic) RFC1475, RFC6814
8 Reserved (Historic) RFC1621, SC589H
9 Reserved (Historic) RFC1347, SC589H
10-14 Unassigned Jon_Postel
15 Reserved Jon_Postel

Looking carefully, I found that IP version 15 was reserved by Jon Postel. I already know the uses of the other versions, but I have never seen or heard of IP Version 15. Why is IPv15 reserved? What is its purpose?

skksky
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  • Interesting question (even if it might be off-topic). Down the rabbit role (https://en.wikipedia.org/wiki/Jon_Postel).. – user2864740 Oct 03 '21 at 02:48
  • Thanks!Of course I know about Jon Postel. He was a great man who wrote the history of TCP/IP. – skksky Oct 03 '21 at 02:53
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    Since originally that field is only supposed to be a single bit https://www.rfc-editor.org/ien/ien26.pdf then quickly changed to 4 bit https://www.rfc-editor.org/ien/ien28.pdf and is the very beginning of the header, reserving 15 which would show up as 1111 might be to detect invalid packet. – Martheen Oct 03 '21 at 03:39
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    I see, the version header is 4 bits, and 1111 in binary is 15 in decimal, so it may have a special meaning in finding invalid packets. – skksky Oct 03 '21 at 04:07

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