Classless Inter-Domain Routing Notation

BGP uses Classless Inter-Domain Routing (CIDR) notation for masks. When advertising routes, BGP will include prefixes in it's advertisements. A prefix is the network IP address plus the mask in CIDR notation.

Below are tables showing how IP masks and CIDR masks are related. In every IP address, certain bits are used to identify the network, and certain bits are used for host. The mask allows the receiver to tell which bits are network, and which are host. Ones are used to mark the Network bits, and zeroes are used to mark the Host bits.

	CLASS 'A' NETWORKS
BINARY	11111111.00000000.00000000.00000000
DECIMAL	255.0.0.0
CIDR	/8

 

	CLASS 'B' NETWORKS
BINARY	11111111.11111111.00000000.00000000
DECIMAL	255.255.0.0
CIDR	/16

 

	CLASS 'C' NETWORKS
BINARY	11111111.11111111.11111111.00000000
DECIMAL	255.255.255.0
CIDR	/24

You will note that the number in the CIDR mask notation is equal to the number of 1's in the binary mask. The same holds true for subnets:

	1/2 CLASS 'C' NETWORK
BINARY	11111111.11111111.11111111.10000000
DECIMAL	255.255.255.128
CIDR	/25

And for supernets as well:

	2 CLASS 'C' NETWORKS
BINARY	11111111.11111111.11111110.00000000
DECIMAL	255.255.254.0
CIDR	/23

So for CIDR notation, the number after the slash is equal to the number of network bits (1's) in theymask.

 

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