Internet Protocol (IP)
Table of Contents
1. Why was there no IPv5?
There was no IPv5. IP version is specified in the first 4 bits of the IP header.
In between IPv4 and IPv6 another protocol called the Internet Stream Protocol (STv1 and STv2) was developed. It was not based on IP but to avoid confusion it used the number 5 in its version field. And thus when another version for IP was developed it used version 6.
2. What about IPv1, IPv2, IPv3?
In case of IPv1, IPv2, and IPv3 there version numbers were internally used during development inside DARPA. And the first public standard was IPv4. [Source]