Many LAN messengers offer basic functionality for sending private messages, file transfer, chatrooms and graphical smileys. The advantage of using a simple LAN messenger over a normal instant messenger is that no active Internet connection or central server is required - and only people inside the firewall will have access to the system.
A precursor of LAN messengers is the Unixtalk command, and similar facilities on earlier systems which enabled multiple users on one host system to directly talk with each other.
On Windows, WinPopup, was a small utility included with Windows 3.11. WinPopup uses SMB/NetBIOS protocol and was intended to receive and send short text messages.
Windows NT/2000/XP improves upon this with Windows Messenger service, a Windows service compatible to WinPopup. On systems where this service is running, the received messages "pop up" as simple message boxes. Any software compatible with WinPopup, like the console utility NET SEND, can send such messages. However, due to security concerns, by default, the messenger service is off in Windows XP SP2 and blocked by Windows XP's firewall.
On Apple's Mac OS X based computers, the iChat program allows LAN messaging over the Bonjour protocol since 2005. The multi-protocol messenger Pidgin has support for the Bonjour protocol, including on Windows.