8.3 BGP Neighbor Negotiation
8.3.6 Network layer reachability information
Rather than advertising reachable destinations as a network and a subnet mask, BGP advertises them using NLRIs, which consist of prefixes and prefix lengths. The prefix represents the reachable destination, while the prefix length represents the number of bits set in the subnet mask. For example, 10.1.1.0 255.255.255.0 has a prefix of 10.1.1.0, and a prefix length of 24 (there are 24 bits set in the subnet mask), and thus would be advertised by BGP as 10.1.1.0/24.

The NLRI consists of multiple instances of the 2-tuples <length, prefix>. The figure illustrates the NLRI <19, 192.24.160.0>. The prefix is 192.24.160.0, and the length is a 19-bit mask (counting from the far left of the prefix).