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Community Member

Classful Vs. Classless Routing protocols

Hi, I am just after a bit of clarification. With Classful routing protocols I have heard a lot of different information. Some people say a network cannot be subnetted, e.g the network 10.1.1.0 /24 is actually advertised as 10.0.0.0 /8 if a Classful routing protocol (such as Rip v1) is used. Others say you can subnet the network address so long as each subnet uses the same subnet mask, e.g the 10.0.0.0/8 network can be subnetted into 10.1.1.0 /24, 10.1.2.0 /24, 10.1.3.0 /24

Could someone please shed some light on this?

Thank you.

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Classful Vs. Classless Routing protocols

A classful routing protocol does not support VLSM/discontiguous networks because they do not advertise the subnet mask with routing updates.

So if you had 10.1.0.0/24 on R1 and 10.2.0.0/24 on R2 and both were advertising to R3 using RIPv1, then R3 would just simply see two routes for 10.0.0.0/8, one from each of the routers.

Hope this helps.

Nik

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Will

yes if subnet 10.1.0.0 was connected to 10.5.0.0 which was connected to 10.2.0.0 it would be classified as contiguous.

No I do not mean to say that the classful routing protocols advertise the subnet mask. The classful routing protocols assume that the same subnet mask will be used on all subnets within the same major network and therefore there is no need to advertise the subnet mask.

As to your last question, I mean that when you create the first subnet within network 10 you can choose any subnet mask that makes sense. But when you create the second subnet (and all other subnets) it is important that they all use the same subnet mask. Lets assume that you are subnetting network 10. So you create 10.1.0.0/24. Then all subnets within network 10 need to use /24 masks. When you create subnets within network 172.16 you can choose a different mask (perhaps a /22) but then all subnets of 172.16 need to use the same subnet mask.

HTH

Rick

13 REPLIES

Re: Classful Vs. Classless Routing protocols

A classful routing protocol does not support VLSM/discontiguous networks because they do not advertise the subnet mask with routing updates.

So if you had 10.1.0.0/24 on R1 and 10.2.0.0/24 on R2 and both were advertising to R3 using RIPv1, then R3 would just simply see two routes for 10.0.0.0/8, one from each of the routers.

Hope this helps.

Nik

Community Member

Re: Classful Vs. Classless Routing protocols

Excellent, that is exactly what I thought. Thank you for the clarification, it is much appreciated.

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Will

The answer from Nik is true if you make one assumption and not true if you make a different assumption. If you assume that R1 and R2 advertise to R3 over some network that is different from 10.0.0.0 (perhaps 192.168.x.x) then the answer given is true. This is because when advertising over some other network (like 192.168) RIPv1 will automatically summarize at network boundaries. So the advertisements from R1 and R2 would advertise 10.0.0.0/8 over 192.168.x.x. But if you assume that R1 and R2 are advertising to R3 over subnets of 10 (perhaps 10.11.0.0/24 and 10.22.0.0/24) then the original subnets would be advertised and R3 would see the original subnets from R1 and R2.

The key to this issues is correctly pointed out by Nik which is that RIPv1 does not support discontiguous subnets.

HTH

Rick

Community Member

Re: Classful Vs. Classless Routing protocols

What exactly does "discontiguous subnets" mean? Does that mean you have to have subnet 1 connected to subnet 2 and subnet 2 connected to subnet 3, or does it mean all subnets must have the same number of borrowed bits?

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Will

contiguous subnets means that subnet1 is connected to subnet2 which is connected to subnet3, etc. discontiguous subnets means that subnet1 is connected to a subnet of some other network, which is connected to some subnet of the original network.

So contiguous subnets would be 10.1.0.0 connected to 10.2.0.0 which is connected to 10.3.0.0. discontiguous subnets wold be 10.1.0.0 connected to 192.168.2.0, which is connected to 10.3.0.0.

Having the subnet masks the same size within a network is also important to classful routing protocols. But that is a different issue than contiguous and discontiguous.

HTH

Rick

Community Member

Re: Classful Vs. Classless Routing protocols

I see. So if subnet 10.1.0.0 was connected to 10.5.0.0 which was connected to 10.2.0.0, is this also classified as contiguous?

Also, in regards to your comment here, "if you assume that R1 and R2 are advertising to R3 over subnets of 10 (perhaps 10.11.0.0/24 and 10.22.0.0/24) then the original subnets would be advertised and R3 would see the original subnets from R1 and R2. " Do you mean to say that routers do send the subnet mask when using a classful routing protcol in a contiguous network?

Also, in regards to the last line of your comment, "Having the subnet masks the same size within a network is also important to classful routing protocols", what do you mean by this? Is it a must? I am assuming so because routing protocols do not support VLSM.

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Will

yes if subnet 10.1.0.0 was connected to 10.5.0.0 which was connected to 10.2.0.0 it would be classified as contiguous.

No I do not mean to say that the classful routing protocols advertise the subnet mask. The classful routing protocols assume that the same subnet mask will be used on all subnets within the same major network and therefore there is no need to advertise the subnet mask.

As to your last question, I mean that when you create the first subnet within network 10 you can choose any subnet mask that makes sense. But when you create the second subnet (and all other subnets) it is important that they all use the same subnet mask. Lets assume that you are subnetting network 10. So you create 10.1.0.0/24. Then all subnets within network 10 need to use /24 masks. When you create subnets within network 172.16 you can choose a different mask (perhaps a /22) but then all subnets of 172.16 need to use the same subnet mask.

HTH

Rick

Community Member

Re: Classful Vs. Classless Routing protocols

Excellent Rick, you have explained your points very clearly, thank you.

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Will

I am glad that my answers were helpful. Thank you for the rating.

I encourage you to continue your participation in the forum.

HTH

Rick

Community Member

Re: Classful Vs. Classless Routing protocols

Not a problem, you deserved it :)

I shall, it is a great forum with a lot of knowledgable people.

Re: Classful Vs. Classless Routing protocols

Rick,

You really deserve more than 5 :) Perfect explaination.

BR,

Mohammed Mahmoud.

Hall of Fame Super Gold

Re: Classful Vs. Classless Routing protocols

Mohammed

Thank you for the compliment. It is good to have many contributors, including you, to maintain the high quality of the responses in the forum.

HTH

Rick

Community Member

Re: Classful Vs. Classless Routing protocols

I have only been using these forums for the last couple of days but I have seen both of you posting with a lot of very informative posts so you both deserve a pat on the back. Keep up the great work.

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