Hi Carl_Townshend,
The difference between classful IP addressing and classless IP addressing is in selecting the number of bits used for the network ID portion of an IP address. In classful IP addressing, the network ID portion can take only the predefined number of bits 8, 16, or 24. In classless addressing, any number of bits can be assigned to the network ID.
1) if the route in question is a member of the same major network number (that is, the same class A, B, or C network part) as a directly connected interface, the mask on that interface is assumed for the route. The implication is that since everyone in the network assumes their local mask is the correct mask for all subnet routes, then all local masks must be configured the same for all subnets of the same major network number.
2) if the route in question is a member of a foreign network (that is,none of the router's directly connected interfaces have a subnet of the same A, B, or C network part), the router assumes the default mask for whatever class address the route represents: 255.0.0.0 for A, 255.255.0.0 for B, 255.255.255.0 for C. It is this constraint that requires all subnets of the same major network number to be contiguous when using a classful routing protocol.
In a classless routing protocol, the netmask is always propogated with the route being advertised. So the consideration of what the netmask is on a router's local interfaces or what class address, A, B, or C; the route represents are no longer important: the router always uses what it learned from the route update and requires no local information to determine the correct netmask for the route.
3)Short answer: Classful protocols cannot carry a network mask as part of the routing advertisement. This limits the routing protocol to only supporting a single subnetting scheme within the autonomous system. Additionally, when advertising network addresses between different major network spaces, the routing protocol will summarize the routing information to the natural class (A, B, C) of the network address. It must do this since there is no method for one major class network to be updated with the network mask from another major class network. The best the routing protocol can do is summarized based on the class of the network (derived from the initial bits of the address) and the natural network mask associated with the network class.
When you are classless, you have to explicitly declare aggregations, since the classful assumptions fly out the window.
You also always have to include masks on IP route statements when running classless.
HTH, Please rate if so
Regards,
Justin
