Bogon filtering is the practice of filtering bogons, which are bogus IP addresses of a computer network. Bogon is also an informal name for an IP packet on the public Internet that claims to be from an area of the IP address space (or network prefix or network block) reserved, but not yet allocated or delegated by the Internet Assigned Numbers Authority (IANA) or a delegated Regional Internet Registry (RIR). The areas of unallocated address space are called the bogon space.
Bogons are not the same as reserved private address and link-local address ranges, such as those in 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, and 169.254.0.0/16, which are reserved for private networks.
Many ISPs and end-user firewalls filter and block bogons, because they have no legitimate use, and usually are the result of accidental or malicious misconfiguration. 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 time goes on, the IPv4 address exhaustion will mean there are fewer and fewer IPv4 bogons.
IANA maintains a list of allocated and reserved IPv4 netblocks.
As of November 2011, the IETF recommends that bogon filters for previously unused IPv4 /8 netblocks should be removed, as all have been assigned.
Etymology
The term bogon stems from hacker jargon, where it is defined as the quantum of bogosity, or the property of being bogus. A bogon packet is frequently bogus both in the conventional sense of being forged for illegitimate purposes, and in the hackish sense of being incorrect, absurd, and useless.