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diff between classful routing and classful network

in most text books a classful network is defined as a setup where all networks have same subnet mask. but when configuring rip, the classful is described as a network using default subnet mask. Rip does not advertise subnet masks. It is said the receiving interface on the router configured with rip assumes the same subnet mask on all the receiving networks. but how is the router able to receive some class B networks and class C networks and able to route with no issues. the receiving int subnet mask is /16. the networks received in advert are 192.168.1.0 , 192.168.2.0  uptil 192.168.10.0  on the same interface. if it takes the sm on them as /16, how is the router able to route the packets correctly. someone help me with clear definitions of classful network and classful routing and the differences between them please

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Re: diff between classful routing and classful network

Krishna

First let us clarify a misunderstanding in your logic. You say:"  classful network is defined as a setup where all networks have same subnet mask." This is not quite true. A better understanding is that a classful network uses the same subnet mask for all of its subnets. But a different classful network is free to use a different subnet mask for its subnets. For example you might have one interface on your router that is configured with address 172.16.1.1 255.255.255.240. In this case all subnets of Class B network 172.16.0.0 would use mask of /28. But another interface on your router might be configued with 172.17.1.1 255.255.255.0. In this case all subnets of Class B 172.17.0.0 will use subnet mask of /24.

You also ask:"how is the router able to receive some class B networks and class C networks and able to route with no issues. the receiving int subnet mask is /16." The assumption about a default subnet mask applies only to the network configured on the receiving interface. So if your router interface was configured with address 172.16.1.1 255.255.255.240. In this case the default subnet mask is /28 and not /16. And the default subnet mask applies only to subnets of the class B network 172.16.0.0. So advertisement of networks 192.168.1.0, 192.168.2.0, 192.168.3.0 would use the default Class C mask of /24. Hopefully that clears up how the router is able to route between those networks.

In a classful routing environment the assumption of default network mask applies only to the network configured on the receiving interface. All other networks will use the natural mask for its class of network (Class A mask is /8, Class B mask is /16, Class C mask is /24).

You might find these links helpful in trying to understand the concepts of classful routing:

http://en.wikipedia.org/wiki/Classful_network

http://wiki.answers.com/Q/How_Classless_and_Classful_Routing_protocol_operate_in_Routing

HTH

Rick

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Re: diff between classful routing and classful network

thanks very much Rick. got clarification.

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