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What is the different between Classful and Classless Routing?

I have been preparing to take my last part of my CCNA test later this month, which is ICND2. It goes over a lot of topics, such as routing protocols, VLANs, spanning tree and a lot of troubleshooting. One of the topics involves classful and classless routing as it relates to VLSM (variable length subnet masks) and an IOS command called no ip subnet-zero. So it got me to thinking What is classful routing?

Let’s talk about classful routing, which is sometimes called a classful network. If you are using a classful routing protocol then all of your networks are in different classes and are separated by a router. Remember that the useable IPv4 address ranges are dived into classes:

  • Class A = 1-126
    (0.0.0.0 – 126.255.255.255)
  • Class B = 128-191
    (128.0.0.0 – 191.255.255.255)
  • Class C = 192-223
    (192.0.0.0 – 223.255.255.255)

When we talk about a classful routing protocol it will only look at the IP address class and not the subnet mask. This in turn will place a restriction on the router on what networks can use the default route. This will result in cases where a router has a default router but chooses to discard the packet rather than forward the packet to the default route.

That is a problem when you look at how we configure networks today. Please keep in mind that if you are using a classful routing protocol, then you can’t change the subnet mask in any of those networks. You have to use the default subnet mask, so an example is if you have a 172.16.X.X network the default subnet mask is a /16 or 255.255.0.0. (over 65,000 hosts) In short you can’t have different size networks when using the same IP address class and a classful routing protocol. It gets worse if you wanted to use the 10.X.X.X network, since you would not be able to cut it down into smaller pieces and would be stuck with over 16 million hosts.

Remember that classful routing does not support subnet information, and therefore it is lacking support for VLSM. Classful addressing always has an 8, 16, or 24 bit network field baced on the Class A, B or C addressing rules. Classless addressing uses a two-part view of an IP address and classful addressing used a three part
view.


With classless routing, you have support for VLSM and different size networks within the same IP address class. This is exactly what a classless routing protocol does, so for example we are using a class C address of 192.168.1.0 with a standard subnet mask of 255.255.255.0 or a /24. With a classless routing protocol you can split this network even more instead of having 254 usable hosts in one network; you could have 126 usable hosts with two networks. The ranges for the first network would start at 192.168.1.1 and end at 192.168.1.126, and the second would start at 192.168.1.129 and end at 192.168.1.254. The subnet mask for both of these networks would be 255.255.255.128. With a classful routing protocol it would only look at the class of the address in this case a class “C” and not look at the subnet mask and apply a default subnet mask of 255.255.255.0 causing these addresses to be in the same network which in reality their not.

What Routing Protocol uses Classful and Classless Routing?


That’s my short summary for classful and classless routing protocols, I wouldn’t say nobody uses a classful routing protocol there’s always someone that has a justification for using this type of networking. For more information on how IPv4 networks are broken up, please see this earlier article that I wrote in June.

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One Response to “What is the different between Classful and Classless Routing?”

  1. I’ve recently found a nice way to have different class IPs working for my services. It’s called rocketips and it seems to be a relief to my classic, hand-made subnets and networks. Results are astonishing.

    November 13, 2013 at 1:37 AM
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