Overview
There have been several attempts attempts to enable communication between MUDs.
Aberchat
Intermud protocol for AberMUDs
Arachnos
A intermud messaging via MSDP
Intermud (CD Intermud)
The one that started it
Circle Intermud
One of the first attempts on inter-mud communication
IMC2
Source 1//www.mudbytes.net/imc2-intermud-protocol-documentation
Intermud1/2
One of the first attempts on inter-mud communication
Intermud3
Architecture
Merentha InterMUD System)
Network Architecture
Mudnet
Source 1: MUdnet Credits
MUDVault Mesh
A websocket based protocol
MUSHLink
MushLink is primarily used on MUSHes. It uses a TCP connection in order to connect a MUD to the central server. In contrast to the other interMUD networks so far listed, MushLink's server actually connects to the MUD instead of having the MUD connect to the server. This connection consists of a bot that logs in as a special user. This user uses special commands in order to both display and receive messages from the MUD.
Realm of Magic Intermud
Source 1: Teaser
Zebedee
Source//mud.stack.nl/intermud/zebedee.protocol.html