Classful vs Classless Routing Protocols

Agree with the comments above. Clear definition. Thanks.


Great explanation. Cheers dude

great help !!! thanks :slight_smile:

Hi Rene,

i have configured rip version 2, i could not see the advertisement of “ subnet with a subnet mask” and subnet with a subnet mask.

Configure “no auto-summary” under router RIP. By default RIP version 2 behaves classfull.

Hi Rene,
In what scenarios we can use classful and classless routing protocols?
Is it beneficial to use RIPv1 in small networks??

Hi Sham Rao,

There is no good reason to use a classful routing protocol nowadays. Keep in mind that some of these concepts are really old. The RFC for RIP for example is from 1988…

We didn’t have things like VLSM back then :slight_smile:


So simple and beautifully explained. Thanks Rene.

Very simple and easy to understand…Thank you


Since you mentioned in the video that RIP is no longer on the CCNA exam, is there anywhere else that classful routing might come up in the exam?

Thanks again, and nicely done,


Hi Chris,

Yes there is, EIGRP also supports classful and classless routing.


If I remember well, the same thing if we put and insteadof 192.168… Because 172.17 … Is another major network… Right??

Hi Francesco,

Those two networks are from the class B range so yes, they would be summarized to

192.168.X.0/24 is no problem btw…this is a class C network.


Hi Rene ,

Is “Discontiguous Network” issue is due mainly classfull routing lookup ? , because it will do look up only classfull manner , like router has to do a look up for destination network /24 , as the 1st octate is belongs to class A range it wont check further the the mask part , if network not existing it will drop it ?

Can you guide on “Discontiguous Network” and its solution aprroach .


Hello Sameer

For the explanation, I will use the subnets used in the lesson.

The problem with classful networks is that if there is a destination of this will be installed in the routing table as since 172.16.X.X is a class B network. This works fine as long as all destinations of are to be routed via the same interface.

However, if you have a case such as this, discontiguous networks with classful routing presents a problem:
Assuming a classful routing protocol is being used, R3 will inform R2 that it has a destination to and R2 will put the following into its routing table:

to reach, route via Fa0/1 with a next hop IP of

But R1 also sends a routing update to R2 stating that it has a route to so R2 will put the following into its routing table:

to reach, route via Fa0/0 with a next hop IP of

During the next routing updates, R3 and R1 will alternately inform R2 that they have routes to these destinations, while all the time R2 is installing the classful network into the routing table, causing incorrect routing to occur.

So, this is why discontiguous networks can cause problems with classful routing.

In order to solve such issues, there are two solutions:

  1. The first involves adjusting your addressing scheme so that networks are not discontiguous, that is, all class A networks can be reached via a single interface, and similarly for class B and class C. This is not always possible and can be somewhat restrictive.
  2. The ideal solution, and the one that is considered best practice, is to use a classless routing protocol so that you can define specific subnets within classful networks and route in a classless manner.

The second choice should always be the selected solution in all modern IP networks.

I hope this has been helpful!


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Thank you Laz!!

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Great explanation, I really get this now, thanks!

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The below from Eigrp module
EIGRP will check if you have any subnets that fall within the range of your network command(s) and if so, it will advertise the classful A, B or C network to its neighbors. It will only summarize routes from subnets on its own interfaces, not the ones you learn from other routers.

In this article you are saying Eigrp is classless

Hello Sims

This statement is found within the following lesson and is explaining how auto-summary functions for EIGRP:

So when auto-summary is enabled, this is the behaviour that we see from EIGRP. It will advertise the classful subnets that correspond to the network commands. Note here that Rene mentioned that:

…EIGRP is not going to automatically create the most optimal summary routes for you.

The purpose of the lesson is to show how auto-summary behaves so that you can either use it correctly or avoid using it and employ a summarization technique that will better suit your network.

When auto-summary is enabled, yes, EIGRP will behave like a classful routing protocol. But when disabled, it is a classless routing protocol.

I hope this has been helpful!