A bogon is an bogus IP address from the bogon space, which is a set of IP addresses not yet officially assigned to any entity by the Internet Assigned Number Authority (IANA) or a regional Internet registration institute.

Bogon IP addresses are legitimate addresses. You may see a bogon IP address as a result of a misconfiguration (whether intentional or unintentional) that fools a recipient about a sender‘s legitimate IP address. Bogon IP addresses are popular in hacking or malicious activities and are used by spammers and those who initiate distributed denial-of-service attacks. As such, many Internet service providers and firewalls block bogons.

A bogon is also known as a bogon space or bogon IP address.