为什么没有ipv5？

有ipv5 的
ipv4 v6 的全称 是 internet protocol version 4 ，
internet protocol version 6，

在维基百科 找到的
IPv5 (Internet Protocol, version 5) was assigned to an experimental protocol called ST (Internet Stream Protocol). ST was first defined in 1979 in IEN 119 (Internet Engineering Note), and was later revised in RFC 1190 (ST2) and RFC 1819 (ST2+). ST was envisioned to be the connection oriented complement to IPv4, but it has never been introduced for public usage. Many of the concepts available in ST can be found today in MPLS.