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Classless and Classful routing

Can someone please explain the differences between them? i can't understand it, no matter how much i read about it...:(


Re: Classless and Classful routing

Re: Classless and Classful routing


Ok, we must differentiate here between classful/classless routing protocols, and classful/classless router forwarding behavior.

The whole idea here is that Classful routing protocols don't advertise the subnet mask with the route, accordingly the router has to assume one when it receives the route via a classful routing protocol (and the router that has advertised the routes must do some checkups before sending it).

Kindly check the following document for the behavior of classful routing protocols

Behavior of RIP and IGRP When Sending and Receiving Updates

On the other hand, Classful route lookups, in which a destination address is first matched to its major network address in the routing table and is then matched to a subnet of the major network. If no match is found at either of these steps, the packet is dropped (A default route will only work if the major classful route is not in the routing table). For earlier IOS versions you can enable classless route lookup, even for classful routing protocols such as RIPv1 and IGRP, by entering the global command ip classless. When a router performs classless route lookups, it does not pay attention to the class of the destination address. Instead, it performs a bit-by-bit best match between the destination address and all its known routes.


Mohammed Mahmoud.


Re: Classless and Classful routing


when a classfull routing protocol is running and if for a route X ,the routing table

only have a major network number entry (ie,no subnet entry),then the packet will be

droped even if a default route is there.....

if a major network entry is not there ,then it would take default route and move...

so in order that default route to work properly with classfull routing protocols(RIPv1,

igrp) we need to give ip classless cmd.

In current IOS ,its enabled by default.... :)

New Member

Re: Classless and Classful routing

So classless routing is the default routing

(forwarding method) in the current IOS?

Re: Classless and Classful routing

Hi Oren,

In the essence of classful/classless routing, yes "ip classless" is there by default in the new codes, but as i said before you must differentiate between classful/classless routing protocols, and classful/classless router forwarding behavior.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

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