Bogon filtering is the practice of filtering bogons, which are bogus (fake) IP addresses of a computer network. Bogons include IP packets on the public Internet that contain addresses that are not in any range allocated or delegated by the Internet Assigned Numbers Authority (IANA) or a delegated regional Internet registry (RIR) and allowed for public Internet use. The areas of unallocated address space are called the bogon space.

Bogons also include some address ranges from allocated space, also known as Martian packets, mainly when they are being used as source address. Addresses reserved for private networks, such as those in 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 and fc00::/7, loopback interfaces like 127.0.0.0/8 and ::1, and link-local addresses like 169.254.0.0/16 and fe80::/64 are part of it. Also addresses for Carrier-grade NAT, Teredo, and 6to4 and documentation prefixes fall into this category.

Many ISPs and end-user firewalls filter and block bogons, because they have no legitimate use, and usually are the result of accidental misconfiguration or malicious intent. Bogons can be filtered by using router access-control lists (ACLs), or by BGP blackholing.

IP addresses that are currently in the bogon space may not be bogons at a later date because IANA and other registries frequently assign new address space to ISPs. Announcements of new assignments are often published on network operators' mailing lists (such as NANOG) to ensure that operators have a chance to remove bogon filtering for addresses that have become legitimate. For example, addresses in 49.0.0.0/8 were not allocated prior to August 2010, but are now used by APNIC. As of November 2011, the Internet Engineering Task Force (IETF) recommends that, since there are no longer any unallocated IPv4 /8s, IPv4 bogon filters based on registration status should be removed. However, bogon filters still need to check for Martian packets.