There are accounts of large botnets, comprised of tens of thousands individual machines acting as one for the sole benefit of the botmaster. Smaller botnets can range from hundreds to a thousand drones. Most computer users do not realize that they are part of the botnet until visible symptoms of infection appear.
Malicious botnets rely on Internet Relay Chat (IRC) bots. IRC is a form of real-time communication using the Internet, and it is mostly designed for group communication. Unlike instant messaging, IRC allows one-to-many communication in channels (a kind of discussion forum). There are extremely many types of IRC-based bots, ranging from extremely simple experiments to ultra-sophisticated functionalities. Regardless of their sophistication level, they are always joining a specific IRC channel on an IRC server, in order to wait for subsequent commands from the botmaster. Some Complex IRC bots also feature their own spreading routines, which allow them to infect other computers connected to the Internet in a similar manner to the worms.