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, and .onion may be included. 
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