Virtual worlds known as MUDs use the telnet protocol to connect. However, the telnet protocol is not currently permitted on Wikidata.org:
Examples of items and desired URL values:
Q1228613 Discworld MUD - telnet://discworld.starturtle.net:23
Q5509719 FurryMUCK - telnet://furrymuck.com:8888
Q12046823 Prahy - telnet://prahy.mmh.cz:3333
Q4999806 BurningMUD - telnet://burningmud.com:4000
This ticket was inspired by T140986, which is a fulfilled request. If there is an alternative way to model this data I would like to know. Thank you!