DCC Direct Client-to-Client
The IRC extension that lets two clients talk directly to each other instead of through the server. The basis of all file transfer on IRC since the late 1980s.
Negotiation happens as an ordinary CTCP message through the server: the sender sends its IP address and a port number, and the recipient connects straight there. After that the server is entirely out of the picture, which gives full speed and no logging — but also exposes the address and requires the recipient to be reachable through the firewall. Passive DCC reverses the roles to solve the NAT problem. DCC CHAT gives a private conversation with no intermediary, DCC SEND gives file transfer. Combined with scriptable file servers this made IRC one of the largest distribution channels for files during the 1990s.