Classful is based on the default Class A,B or C networks.
– Class A: 0 – 127 with a mask of 255.0.0.0 (/8)
– Class B: 128 – 191 with a mask of 255.255.0.0 (/16)
– Class C: 192 – 223 with a mask of 255.255.255.0 (/24)
All devices in the same routing domain must use the same subnet mask. Since routers running a classful routing protocol do not include subnet mask information with routing updates, the router assumes either its own subnet mask, or defaults to the classful subnet mask.
Classless on the other hand, allows the use of variable length subnet masks, or VLSM, because subnet mask information is included with routing updates. You can have a mixture of different subnet masks in the same routing domain: