5 Replies Latest reply: Jun 18, 2012 12:55 AM by Sey RSS

    172.31.0.0/24 is subnetted ,10 subnets  what does this mean?

    swaminathan

      Hi

       

      In ICND2 I have found this theory in one example for show ip route.....Gateway of last resort is not set, 172.31.0.0 /24 is subnetted , 10 subnets and it is giving the subnets as 1. 172.31.130.0 .172.31.140.0,..........

       

      What is my confusion is 172.31.0.0 /24 is subnetted means the subnetting should start after 24 bits .? the subnets are in 3rd octet ..

       

      May be I am wrong in my interpretation ! Please anybody kindly explain this

       

      Thanks & Regards

       

      S.Swaminathan

        • 1. Re: 172.31.0.0/24 is subnetted ,10 subnets  what does this mean?
          Sey

          I agree that this is confusing. I'll try to explain that.

           

          The classful network is 172.31.0.0/16, so if you had a subnet with any other prefix length (let it be 172.31.250.32/27) in the routing table, you would see something like

           

          172.31.0.0/16 is variably subnetted, 11 subnets, 2 masks

          172.31.130.0/24 ...

          172.31.140.0/24 ...

          ...

          172.31.250.32/27 ...

           

          However, when there is only a single prefix length within a classful network like in your example, the output of show ip route looks a bit different. The header of the classful network specifies the prefix length, and the following network addresses don't have the prefix length:

           

          172.31.0.0/24 is subnetted, 10 subnets

          172.31.130.0 ...

          172.31.140.0 ...

          ...

           

          As you can see it's not about subnetting but just a notation.

          • 2. Re: 172.31.0.0/24 is subnetted ,10 subnets  what does this mean?
            swaminathan

            Hi Sey

             

            Thank you very much for your explanation and in the example  is clear below and what is the subnet masks for these subnets?

             

             

             

             

            • 3. Re: 172.31.0.0/24 is subnetted ,10 subnets  what does this mean?
              Sey

              It's 255.255.255.0 because of /24 in the header "172.31.0.0/24 is subnetted"

              • 4. Re: 172.31.0.0/24 is subnetted ,10 subnets  what does this mean?
                swaminathan

                Hi Sey

                 

                That means 172.31.0.0 /24 is not a classful network.It should be a classless

                 

                and you have mentioned in your previous post "

                However, when there is only a single prefix length within a classful network like in your example, the output of show ip route looks a bit different. The header of the classful network specifies the prefix length, and the following network addresses don't have the prefix length:

                 

                172.31.0.0/24 is subnetted, 10 subnets

                172.31.130.0 ...

                172.31.140.0 ..."

                 

                You are meaning that it comes under classful or classless network?

                • 5. Re: 172.31.0.0/24 is subnetted ,10 subnets  what does this mean?
                  Sey
                  That means 172.31.0.0 /24 is not a classful network.It should be a classless

                  It's not a classful network but a prefix within a classful network. But in the header it's not about that. It means that the following subnets will all have the same prefix length, and that is /24.

                  and you have mentioned in your previous post "

                  However, when there is only a single prefix length within a classful network like in your example, the output of show ip route looks a bit different. The header of the classful network specifies the prefix length, and the following network addresses don't have the prefix length:

                   

                  172.31.0.0/24 is subnetted, 10 subnets

                  172.31.130.0 ...

                  172.31.140.0 ..."

                   

                  You are meaning that it comes under classful or classless network?

                  I mean classful. 172.31.0.0/24 in the header means that there is no other prefix length within the classful 172.31.0.0/16 network. Otherwise it would read "172.31.0.0/16 is variably subnetted, XX subnets, YY masks".

                   

                  I repeat, this has nothing to do with subnetting. It's just a notation that Cisco chose to use.