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So now let’s look at the difference between

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a classful and a classless routing protocol.

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Firstly, classful routing protocol do not advertise

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the subnet mask as an example, they would advertise network 10.0.0.0

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but would not inform other routers of the subnet mask.

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So other routers wouldn’t know if this is a /8 or /24 mask.

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Now this becomes very important in routing because for example

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if I had a PC configured with an IP address of 10.1.1.1

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and your Pc had an IP address of 10.1.2.1 are our PCs in the same subnet?

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Well that very depends because if the subnet mask used is /8

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then our PC's are on the same subnet but if the mask used is /24

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then our PC's are not in the same subnet.

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This makes routing very difficult

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so what classful routing protocols do

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is they assume that everyone else is using the same subnet mask as they are.

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So in other words, if they received a routing update

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with a network of 10.0.0.0 on a interface

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where the IP addresses configured with /24 mask

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the router will assume that other routers will also using with /24 mask.

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Now we all know that making assumptions can be very, very dangerous.

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What this means is that routers assumed network consistency

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in other words, that the same mask is used in the network.

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The routers assume that everyone else is using the same subnet mask

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as they are and that all mask are the same.

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In other words, everyone is using a /24 mask

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that’s not scalable or deployable in networks today

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so in today's networks, we don’t use classful routing protocols such as RIP version 1.

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classful routing protocols also summarized routes across classful boundaries.

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So when you go from a class A subnet like 10.0.0.0

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to a class B subnet like 172.16.0.0

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the routers will automatically summarize all subnets to the classful boundary.

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In other words a subnet of 10.1.1.0/24 will be summarize to 10.0.0.0/8

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example of classful routing protocols include RIP version 1 and IGRP

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IGRP is no longer used and you shouldn’t be using RIP version 1.