An IP prefix is a block of internet addresses described by a network address and a mask, such as 203.0.113.0/24. Routers use prefixes to decide where traffic should go, so a prefix is the basic unit of reachability on the internet. If a route is advertised for a prefix, networks may send packets toward the system that claims to own or carry that address space.
IP prefixes matter in security because attackers can abuse routing to misdirect traffic. In a BGP hijack, a malicious or mistaken announcement for a prefix can pull traffic away from the real destination, enabling disruption, interception, or confusion during incident response. Defenders monitor prefix announcements, use origin validation and RPKI, and apply prefix filters to reduce the risk that unauthorized routes will be accepted.