Network identification

User is identified on network using these 3 strings which are written as nick!ident@hostname by server

Nick

User configurable unique identifier. There can't be 2 same nicknames on same network. Nickname can be changed during the session

Ident

Comes from ancient times when multiple POSIX users shared same system (server). If there are multiple users connected to a POSIX system which is running identd (usually as root user), which is listening on its network port, application on other side (IRC server) can verify if the application that connected to it is really running under the POSIX user account that it claims to run as. So that POSIX user "james" that run the IRC client would have ident of "james", and this ident could be used to distinguish his session from sessions of other users on same system, who would have same hostname.

Not all IRC servers support ident verification, but these that do usually prefix unverified idents with ~ symbol. Nowadays ident is mostly a user defined string that doesn't have any particular meaning.

See https://en.wikipedia.org/wiki/Ident_protocol for more

Hostname

Configured by server, used to show user hostname as resolved using DNS PTR (reverse IP lookup), but nowadays often consist of various user defined cloaks. Can be changed during the session by server.