A pseudo-top-level domain is a label or name for a computer network that is not participating in the world-wide official Domain Name System and may not even participate in the Internet, but may use a similar domain name hierarchy. Historically the best known large networks in this group were .bitnet, .csnet, .oz, and .uucp, for which many Internet mail forwarders provided connectivity. In addition, newer networks like .exit, .i2p, and .onion may be included. 
- Grothoff, Christian and Wachs, Matthias and Wolf, Hellekin and Appelbaum, Jacob (2014-03-03). "Special-Use Domain Names of Peer-to-Peer Systems". Internet Engineering Task Force. Retrieved 2012-07-03.
- "openissues.ca". openissues.ca. 2012-02-19. Retrieved 2012-06-30.
- Frey, Donnalyn; Adams, Richard L. (1994). "Pseudo Top-Level Domains". !%@:: a directory of electronic mail addressing & networks. In a Nutshell Series (4th ed.). O'Reilly & Associates. pp. 10–11. ISBN 978-1-56592-046-0.