Internet Relay Chat

Internet Relay Chat (usually given as the acronym, IRC) is both a protocol and a program suite.

Versions

IRC II

I think this is the oldest one that'll still work on modern servers. The source that I usually look for is ircii-4.4

irc2.1

As far as I'm aware, this is the oldest version of the IRCd. The protocol is out of date, no current client can talk to it, other than the bundled one. I think it may be wise to try and run it on 2.11BSD or similar. See here for sources.