Internet Relay Chat (IRC) is an application layer protocol that facilitates communication in the form of text. The chat process works on a client/server networking model. IRC clients are computer programs that a user can install on their system or web based applications running either locally in the browser or on 3rd party server. These clients communicate with chat servers to transfer messages to other clients.[1] IRC is mainly designed for group communication in discussion forums, called channels,[2] but also allows one-on-one communication via private messages[3] as well as chat and data transfer,[4] including file sharing.[5]

Client software is available for every major operating system that supports Internet access.[6] As of April 2011, the top 100 IRC networks served more than half a million users at a time,[7] with hundreds of thousands of channels[7] operating on a total of roughly 1,500 servers[7] out of roughly 3,200 servers worldwide.[8] IRC usage has been declining steadily since 2003, losing 60% of its users (from 1 million to about 400,000 in 2012) and half of its channels (from half a million in 2003).[9]

As of March 2018 the largest IRC networks are:

freenode – around 99k users at peak hours
IRCNet – around 44k users at peak hours
QuakeNet – around 36k users at peak hours
EFnet – around 26k users at peak hours
Undernet – around 25k users at peak hours
Rizon – around 25k users at peak hours
AnonOps – around 30k users at peak hours
ChatAmigos – around 24k users at peak hours