I know that IPv6 is the successor of IPv4. But I don't understand versioning policy. Why not IPv5? Did IPv1, IPv2, and IPv3 exist?
As noted elsewhere, IP version 5 was assigned to Internet Stream Protocol.
But IP versions actually start at 0, not 1. IPv0 was described in IEN 2. What might be called IPv1 was described in IEN 26. It called for a one-bit version field, which seems shortsighted today. IPv2 was described in IEN 28. These IP versions were experimental and never gained wide use.
What may also surprise you is that IP versions 7 through 9 have also already been defined. These were three other competing protocols, TP/IX, PIP and TUBA, respectively, which were invented around the same time as what became IPv6 and also intended to replace IPv4. If IPv6 ever needs to be replaced, we'll start at IPv10...
Version number 5 was already used for the experimental Internet Stream Protocol when the next-generation IP protocol was devised, so it became IPv6.