Bogon filtering

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 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.

...
Wikipedia