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, and uucp, for which many Internet mail forwarders provided connectivity. In addition, newer networks like exit, i2p, onion, oz, .bit and Freenet may be included.
Some of these networks may use a naming syntax and hierarchy like the official top-level domains and they may serve the same function in creating names for network endpoints. They are used only for special purposes, typically for addressing hosts that were not reachable via the Internet Protocol for use in services such as E-mail and Usenet via UUCP.
Although these networks or domain names have no official status, some are generally regarded as having been unofficially grandfathered, and are unlikely ever to be allocated as top-level domains.
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. ISBN978-1-56592-046-0.