This server split off from the part of the net you were on. Note that netsplits are all from a point of view of the user. After a netsplit rejoins people might say to you “where did you go?” — because from their perspective, *you* split off.
– irc-faq; Version 1.53; November 28, 1995.
Information about general Internet network connections can be found in the section on network connection problems. This page discusses the situation where an overloaded IRC network splits in two.
An IRC network is made up of potentially dozens of servers communicating messages back and forth as fast as possible in order to appear like a single server. Sometimes a network will split into two networks in order to balance its communications load, right in the middle of thousands of conversations.
If you experience a network split, you might well lose communications with some channels or users that end up in the other half of the split. There is nothing you can do about a network split, so carry on chatting and wait to see if the network rejoins itself.