Stands for 'Client to Client Protocol'. Used in IRC to allow one IRC client to send a message to another IRC client. The receiving client can then respond with appropriate information.

CTCP is used over IRC to send commands directly to another's IRC client, rather than to the user. As such, it is typically used for retreiving version or time information from the receiving client. In addition, it is often used in the triggers of fserves and XDCCs, so that the typing of the trigger does not disturb the rest of the channel.

The syntax of sending CTCP commands is quite simple, being merely "/ctcp username command". For example, "/ctcp bob123 VERSION" will return the version of bob123's IRC client. Most IRC clients will automatically reply to a variety of CTCP commands, with TIME and VERSION simply being two of the most commonly used. For information regarding how it is used in the triggers of file servers, refer to the nodes fserve and XDCC for in-depth details its use in each type of server.