Ripv2 and VLSM and CIDR

Because classless routing protocols like RIPv2 can carry both the network address and the subnet mask, they do not need to summarize these networks to their classful addresses at major network boundaries. Therefore, classless routing protocols support VLSM. Routers using RIPv2 no longer need to use the inbound interface's mask to determine the subnet mask in the route advertisement. The network and the mask are explicitly included in each and every routing update.
In networks that use a VLSM addressing scheme, a classless routing protocol is essential to propagate all of the networks along with their correct subnet masks. Looking at the output from debug ip rip for R3 in the figure, we can see that RIPv2 include the networks and their subnet masks in its routing updates.
Also notice in the figure that we have once again added the R4 router in the topology. Remember, with RIPv1, R3 would only send R4 the routes that had the same mask as the FastEthernet 0/0 exit interface. Because the interface is with a /24 mask, RIPv1 only included subnets with a /24 mask. The only route that met this condition was
However, with RIPv2, R3 can now include all of the subnets in its routing updates to R4, as shown in the debug output in the figure. This is because RIPv2 can include the proper subnet mask with the network address in the update.

One of the goals of Classless Inter-Domain Routing (CIDR) as stated by RFC 1519 is "to provide a mechanism for the aggregation of routing information." This goal includes the concept of supernetting. A supernet is a block of contiguous classful networks that is addressed as a single network. On the R2 router, we configured a supernet - a static route to a single network that is used to represent multiple networks or subnets.
Supernets have masks that are smaller than the classful mask (/16 here, instead of the classful /24). For the supernet to be included in a routing update, the routing protocol must have the capability of carrying that mask. In other words, it must be a classless routing protocol, like RIPv2.
The static route on R2 does include a mask which is less than the classful mask:
R2(config)#ip route Null0
In a classful environment, the network address would be associated with the class C mask /24, or In today's networks, we no longer associate network addresses with classful masks. In this example, the network has a /16, or, mask. This route could represent a series of networks or any number of different address ranges. The only way this route can be included in a dynamic routing update is with a classless routing protocol that includes the /16 mask.


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