As all of us know that after IPv4 it came IPv6. How this transition happened?

I just want to know was there any IPv5 also? or there is any other logic in naming this version of IP as IPv6?

  • 5
    I used to think IPv6 would support six address spaces instead of four like in IPv4. Turns out they multiplied it by 4 instead.
    – Joe Z.
    Commented Jan 31, 2013 at 14:14
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    @EvanPlaice: After NCP, there was TCP, which had a version 1 and version 2. When it became clear the protocol needed to be split, version 3 became IPv3 and TCPv3. Both were declared stable at v4, and are protocols are what you're familiar with today. Because TCPv4 doesn't have to be run across IPv4, that protocol remains the same and IP has gone on to v6.
    – Blrfl
    Commented Feb 2, 2013 at 0:39
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    In the olden days odd numbers usually represented beta releases (like the Internet streaming protocol was)
    – Sylwester
    Commented Jun 9, 2013 at 23:12
  • 3
    IP versions 7, 8 and 9 were also assigned to potential IPv4 replacements, so if anything comes after IPv6 it will begin with IPv10. Commented Jul 30, 2013 at 4:26
  • 1
    @nawfal IPv6 is only nominally 128-bit; an IPv6 address is comprised of a 64-bit network part and 64-bit host part. I don't think we can help you with feeling uneasy about having all that unused space. It's meant to be that way, so that nobody has any realistic chance of running out of addresses on a single network segment, or running out of subnets. (Though ISPs still need some education on the latter...) Commented Jan 8, 2016 at 22:48

4 Answers 4


According to Wikipedia, Internet Protocol Version 5 was used by the Internet Stream Protocol, an experimental streaming protocol.

The second version (of Internet Stream Protocol), known variously as ST-II or ST2, distinguishes its own packets with an Internet Protocol version number 5, although it was never known as IPv5.

The Internet Stream Protocol family was never introduced for public use, but many of the concepts available in ST are similar to later Asynchronous Transfer Mode protocols and can be found in Multiprotocol Label Switching (MPLS). They also presaged Voice over IP.


The version numbers for the 'version' are part of the IP header field (described in RFC 791) and is 4 bits wide. As with many of the numbers that find their way into the internet, the numbers for the version are part of the Internet Assigned Numbers Authority.

The list of the version numbers can be found at http://www.iana.org/assignments/version-numbers/version-numbers.xhtml which shows:

 0         Reserved                      https://www.rfc-editor.org/rfc/rfc4928 section 3
 1         Reserved                      https://www.rfc-editor.org/rfc/rfc4928 section 3
 2         Unassigned
 3         Unassigned
 4 IP    - Internet Protocol             https://www.rfc-editor.org/rfc/rfc791
 5 ST    - ST Datagram Mode              https://www.rfc-editor.org/rfc/rfc1190
 6 IPv6  - Internet Protocol version 6   https://www.rfc-editor.org/rfc/rfc1752
 7 TP/IX - TP/IX: The Next Internet      https://www.rfc-editor.org/rfc/rfc1475
 8 PiP   - The P Internet Protocol       https://www.rfc-editor.org/rfc/rfc1621
 9 TUBA  - TUBA                          https://www.rfc-editor.org/rfc/rfc1347
10         Unassigned
11         Unassigned
12         Unassigned
13         Unassigned
14         Unassigned
15         Reserved

And this is where the numbers come from and whats already out there. If there is something after IPv6 that is not one of the already defined numbers, the next available internet protocol version number available is 10.

The specifics of ST can be read in RFC 1190. This protocol was developed by Jim Forgie and was never more than experimental. The wikipedia page about it can be found at Internet Stream Protocol.

Note that these assigned numbers were from days back when things were a bit more... care free with the internet. Classful network /8 blocks were given out fairly freely (known as 'class A' networks) - a number of universities have network spaces of millions (16.7M) of IPv4 addresses.

Allocating versions numbers to experimental protocols was probably also a sign of the times (though IPv6 has made it to practical use). IANA is much more conservative with assigning numbers today.


"So what happened to IPv5? IPv5 was used to define an experimental real-time streaming protocol. To avoid any confusion, it was decided to not use IPv5 and name the new IP protocol IPv6. " (Cisco CCNA Exploration Courses - Accessing the WAN)

Here's a link! @ Hemant You will find there enhancements that IPv6 offers.

  • Nice quote. Could you add a link to it? That would really help this answer. Commented Feb 1, 2013 at 23:17

IPv5 was used to define an experimental real-time streaming protocol. To avoid any confusion, it was decided to not use IPv5 and name the new IP protocol IPv6. Another thing is that IPv6 has a high span of IP adresses that is up to 340 trillion trillion trillions.

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