IP Prefix (CIDR Block)
Also known as: CIDR block, subnet, IP block
A block of contiguous IP addresses written in CIDR notation, such as 192.0.2.0/24 - the unit of routing in BGP.
A prefix is a range of IP addresses expressed as a base address and a mask length, e.g. 192.0.2.0/24 (256 addresses) or 2001:db8::/32. The number after the slash is how many leading bits are fixed; the rest identify hosts within the block.
Networks announce prefixes into BGP to say 'this address space is reachable through me.' A more-specific prefix (a longer mask covering part of a larger block) always wins in routing, which is why more-specific announcements are both a traffic-engineering tool and a hijacking technique.
Every prefix in the routing table has an origin ASN, an RPKI validation state, and often IRR route objects and geolocation data attached to it.