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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).
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