We've got IPv4 and IPv6, yet nobody really talks about IPv5

In the world of networking, most people are familiar with IPv4. These numerical labels, like 192.168.2.1, have identified billions of devices on the internet for decades and have been responsible for connecting those devices to the internet for decades at this point. As of 2025, less than half of Internet traffic is over IPv6, meaning IPv4 still carries the majority of data despite its fundamental limitation of using 32-bit addresses. IPv4 has been on life support for years on account of its 4.3 billion address limit, leading to the advent of IPv6 with its 128-bit address space.

With that said, you may wonder why IPv4's replacement is called "IPv6". Was there an IPv5? The answer is yes... kind of. It was never poised to replace IPv4 (unlike how IPv6 was), and it still used the same 32-bit addressing scheme as IPv4, meaning it did not expand the address space. Its official name is Internet Stream Protocol (ST), and when ST2 was published with RFC 1819, it distinguished its own packets from IPv4 with the version number "5" in the version field of the IP header. Later down the line, when an IPv4 successor was deemed necessary, IPv6 was named as it was to avoid any version number overlap or confusion that would be caused by it.

As a primer to understanding IP versioning, RFC 1700 is responsible for assigning protocol parameters for IP versions, which include version numbers. IPv0 was reserved, IPv1 is unassigned (remaining reserved according to IANA), and both IPv2 and IPv3 are unassigned as well. At some point in time, those numbers did have assignments, as denoted by RFC 755.

What was IPv5?

It never really existed

The truth is a little complex when talking about IPv5, as it didn't really exist, nor did it ever become a mainstream standard. The name "IPv5" has retroactively been informally associated with the experimental Internet Stream Protocol, dating back to the late 1970s. As already mentioned, the version number in the IP header was represented by the number "5", thus suggesting that it was the fifth iteration of the Internet Protocol. This was made more confusing as when ST2 was encapsulated in IPv4 packets, the version would still be 5. It was never intended to be a "successor" to IPv4, but the designers of IPv6 sought to avoid any confusion between IP versions. As a result, it made sense to jump to IPv6 to avoid it; otherwise, you would have two very different IP versions that both identify as version 5 in the packet header.

Internet Stream Protocol (and its later revision, Stream Protocol version 2, or ST2) was a project to enable real-time streaming of voice and video over networks. It was essentially a specialized streaming transport built to run alongside IPv4, and that's why it continued to use the same address space as IPv4. The Stream Protocol was designed to provide features like bandwidth reservation and Quality of Service (QoS) for multimedia communications, especially useful for improving the performance of voice and video calls. The idea was that ordinary data traffic would use IPv4 as usual, while time-sensitive streams could use this new protocol to get special treatment across the network. It was a niche solution for handling streamed media, rather than an overall upgrade to IPv4.

However, this streaming protocol never saw wide public deployment for several reasons. It remained an experimental effort largely confined to research networks and specific projects. ST2 was used in testbeds like the early Defense Simulation Internet for distributed simulations and in Canada's Iris Digital Communications System for digital voice communication, though it didn't really catch on outside of those environments. It still used IPv4 addressing, and adopting a separate streaming-specific protocol like IPv5 alongside IPv4 would have added complexity without addressing the fundamental scalability issue. As we know now, IPv4 is completely fine for streaming, and both QoS and bandwidth reservation can be implemented at the network level through a router, without the need for an additional protocol.

To put all this simply, by the time a true successor to IPv4 was ready, the number 5 was off the table. Many ideas from ST foreshadowed the needs of modern internet applications. For example, it introduced the notion of reserving network resources for high-priority traffic and handling real-time voice data, concepts that would later reappear in other forms and can now be deployed as a feature of a regular consumer router.

ST still influences the internet decades later

Concepts introduced with ST are still used today

Despite never taking off, ST was arguably well ahead of its time in terms of what it delivered. For example, elements of its design were similar to techniques used in Asynchronous Transfer Mode (ATM), which allows for traffic to have defined characteristics such as voice or video. Plus, many concepts went on to be used in Voice over IP (VoIP) applications decades later. It's pretty funny in the sense that ST identified the need for streaming and QoS on the internet long before video calls and streaming media became everyday services.

Even more interesting is that many modern streaming and real-time communication protocols (such as the RTP/RTSP media streaming suite, or the QoS frameworks in networks found today) all appear to essentially achieve the goals of early ST. It may not be used today, but it was one of the first attempts at facilitating real-time communication, and it did work for that at the time.

IPv4 remains the long-lived foundation that's still in use today, whereas IPv6 is the ambitious upgrade desperately trying to address IPv4's limits. Meanwhile, IPv5 was an experimental detour that quietly influenced how we handle streaming data. The version number jumped from 4 to 6 simply because IPv5 was a trial that never became an official standard, and even referring to it as "IPv5" is arguably a misnomer. Still, you'll never see IPv5 in your network's settings, but lessons learned from it are still carried forward decades later in modern networks.