So now let’s look at the difference between
a classful and a classless routing protocol.
Firstly, classful routing protocol do not advertise
the subnet mask as an example, they would advertise network 10.0.0.0
but would not inform other routers of the subnet mask.
So other routers wouldn’t know if this is a /8 or /24 mask.
Now this becomes very important in routing because for example
if I had a PC configured with an IP address of 10.1.1.1
and your Pc had an IP address of 10.1.2.1 are our PCs in the same subnet?
Well that very depends because if the subnet mask used is /8
then our PC's are on the same subnet but if the mask used is /24
then our PC's are not in the same subnet.
This makes routing very difficult
so what classful routing protocols do
is they assume that everyone else is using the same subnet mask as they are.
So in other words, if they received a routing update
with a network of 10.0.0.0 on a interface
where the IP addresses configured with /24 mask
the router will assume that other routers will also using with /24 mask.
Now we all know that making assumptions can be very, very dangerous.
What this means is that routers assumed network consistency
in other words, that the same mask is used in the network.
The routers assume that everyone else is using the same subnet mask
as they are and that all mask are the same.
In other words, everyone is using a /24 mask
that’s not scalable or deployable in networks today
so in today's networks, we don’t use classful routing protocols such as RIP version 1.
classful routing protocols also summarized routes across classful boundaries.
So when you go from a class A subnet like 10.0.0.0
to a class B subnet like 172.16.0.0
the routers will automatically summarize all subnets to the classful boundary.
In other words a subnet of 10.1.1.0/24 will be summarize to 10.0.0.0/8
example of classful routing protocols include RIP version 1 and IGRP
IGRP is no longer used and you shouldn’t be using RIP version 1.