Why is there no IPv5?
There is no general-purpose Internet Protocol called IPv5 between IPv4 and IPv6. The number 5 was already assigned to the experimental Internet Stream Protocol, usually called ST or ST2. It was designed for stateful, resource-reserved streams such as real-time audio and video, not as a drop-in replacement for IPv4.
Internet Protocol version numbers are identifiers in a four-bit header field, not product release numbers that must describe one linear family. Version 5 identified ST2; version 6 identifies IPv6. The gap therefore records protocol history rather than a missing consumer release.
What was IP version 5?
RFC 1190, published in October 1990, specified the experimental Internet Stream Protocol Version 2, or ST-II. It says ST used IP version number 5 and maintained per-stream state so applications could request bandwidth and delay characteristics for real-time traffic.
RFC 1819 revised that work as ST2+ in August 1995. The RFC explicitly says ST2 was experimental, was not an Internet Standard, and was not under consideration for standards status. The RFC Editor now classifies it as Historic.
| Version value | Protocol | Purpose and status |
|---|---|---|
| 4 | IPv4 | General-purpose Internet Protocol using 32-bit addresses; still widely deployed. |
| 5 | ST/ST2 | Experimental stream protocol for resource-reserved real-time traffic; now Historic. |
| 6 | IPv6 | General-purpose Internet Protocol using 128-bit addresses; specified by Internet Standard STD 86. |
The current IANA IP Version Numbers registry lists value 5 as Reserved (Historic). That registry is the authoritative place to distinguish a header version assignment from marketing names or informal protocol histories.
Why did IPv6 use version number 6?
During the 1990s, the IETF evaluated candidates for the next general-purpose Internet Protocol under the IP Next Generation, or IPng, process. RFC 1752, published in January 1995, recorded the recommendation and states that IPng was assigned version number 6. Using 6 avoided the version value already associated with ST2 and gave the selected protocol an unambiguous packet-header identifier.
The first IPv6 specification followed in RFC 1883 in December 1995. The current core specification is RFC 8200, published in July 2017 as Internet Standard STD 86. IPv6 was designed as the general-purpose successor to IPv4, including a 128-bit address space; ST2 was built for a different service model.
IPv5 and IPv6 timeline
- September 1981: RFC 791 publishes the defining early IPv4 specification with version value 4.
- October 1990: RFC 1190 specifies experimental ST-II using version value 5.
- January 1995: RFC 1752 records the IPng recommendation and assignment of version number 6.
- August 1995: RFC 1819 revises ST2 as the experimental ST2+ protocol.
- December 1995: RFC 1883 publishes the first IPv6 specification.
- May 2016: the RFC Editor changes RFC 1819 from Experimental to Historic.
- July 2017: RFC 8200 replaces RFC 2460 and specifies IPv6 as Internet Standard STD 86.
Does IPv5 exist today?
Version value 5 still exists as a reserved historic assignment, but there is no general-purpose IPv5 network for users to migrate to, no IPv5 address format to configure between IPv4 and IPv6, and no public IPv5 address market. Current Internet addressing and migration plans work with IPv4 and IPv6.
A network can run IPv4, IPv6, or both. Dual stack, translation, tunnels, application gateways, and provider-specific transition services can connect those environments, but none of them turns traffic into IPv5. Reachability, security, DNS, routing, and application support must be tested for the actual protocol and path in use.
What this history means for network planning
- Do not wait for IPv5. Plan current services around IPv4 requirements and IPv6 adoption.
- Separate protocol from address supply. IPv6 expands address capacity, while an IPv4 service may still need provider-assigned, leased, or transferred space during a staged transition.
- Test the application path. A protocol label does not guarantee DNS behavior, route acceptance, firewall policy, performance, or third-party compatibility.
- Keep evidence current. For public IPv4, verify registry authority, origin ASN, RPKI, IRR, reverse DNS, reputation, abuse handling, renewal, and return. For IPv6, verify allocation, routing, filtering, DNS, monitoring, and fallback behavior.
For the wider sequence of specifications, read the history of IP addresses. To compare the protocols used today, see IPv4 versus IPv6.
IPv5 FAQ
Did IPv5 ever exist?
The version value 5 was used by the experimental ST/ST2 stream protocol. It was not a deployed general-purpose Internet Protocol positioned between IPv4 and IPv6.
Was ST2 meant to replace IPv4?
No. ST2 was designed to coexist with IP and provide stateful, resource-reserved streams for real-time traffic. RFC 1819 says it was experimental and not under consideration as an Internet Standard.
Why did the IETF skip from IPv4 to IPv6?
The number 5 was already associated with ST2. The protocol selected through the IPng process received version number 6, which became the packet-header value and name IPv6.
Can I get or configure an IPv5 address?
No general-purpose IPv5 address format or public IPv5 network is available. Configure and operate IPv4, IPv6, or an approved transition mechanism that connects those protocols.
Is IPv6 just IPv5 with a larger address space?
No. IPv6 is a separately specified general-purpose protocol with a different header, 128-bit addresses, extension headers, neighbor discovery, and other architectural changes. ST2 used version value 5 for a different stream-oriented purpose.
