Network Layer Reachability Information (NLRI) Field

Network Layer Reachability Information (NLRI) Field
With BGP version 4 came the support for classless interdomain routing (CIDR). BGPv4 is able
to advertise routes regardless of classful boundaries. BGPv4 accomplishes this through the use
of the Network Layer Reachability Information (NLRI) field.
The NLRI field is a variable-length field, which contains the IP address prefix of the route.
The NLRI field consists of a 1-byte Length field and a variable-length Prefix field:
Length The Length field indicates the length of the IP address prefix. You can think of the Length
field as being similar to a subnet mask. By knowing the subnet mask, you are able to determine what
the network address is. The same is true for the Length field. If the value for the Length field is set
to 0, this indicates that all IP addresses are included.
Prefix The Prefix field is of variable length and contains the actual IP address prefix.