Initially, most IRC servers formed a single IRC network, to which new servers could join without restriction, but this was soon abused by people who set up servers to sabotage other users, channels, or servers. In August 1990, the server eris.berkeley.edu remained the only one to allow anyone to connect to the servers.
A group of IRC server operators, with the support of Jarkko Oikarinen, introduced a new "Q-line" into their server configurations, to "quarantine" themselves away from eris by disconnecting from any subset of the IRC network as soon as they saw eris there.
For a few days, the entire IRC network suffered frequent netsplits, but eventually the majority of servers added the Q-line and effectively created a new separate IRC net called EFnet (Eris-Free Network); the remaining servers who stayed connected to eris (and thus were no longer able to connect to EFnet servers) were called A-net (Anarchy Network). A-net soon vanished, leaving EFnet as the only IRC network.
Continuing problems with performance and abuse eventually led to the rise of another major IRC network, Undernet, which split off in October 1992.
Between May and July 1996 IRCnet was formed as a European fork of EFnet, when a number of operator disagreements resulted in a group of European admins declaring their independence. The reasons for the "Great Split" as it came to be called, included:
a policy disagreement about how much power IRC operators should have. IRCnet formed with the basis that there should be a set of rules defining what IRCops could and could not do. This viewpoint was opposed by many of the US-based EFnet servers.
a technical disagreement on whether the network should use timestamping (TS) or Nick Delay as a means to prevent nick collisions, according to Jarkko Oikarinen.
Vegard Engen, one of the European operators, stated that the immediate cause for the "Great Split" was that a major US EFnet hub had been disconnecting irc.stealth.net without warning, and thereby breaking the link to the European servers.