5 Replies Latest reply: Apr 9, 2009 4:00 AM by chris0161 RSS

    SLSM: When to use the zero subnet?

    chris0161

       

      Hello Everyone

       

       

      I am studying subnetting for my CCENT exam and I was wondering if anyone could help out. I am uncertain when to use the zero subnet when deciding which subnets from an available range to use. Here is an example question:

       

       

      An engineer has to assign the first IP address from subnets 3 and 4 in network 172.168.0.0/19 onto ports Fa0/0 and Fa0/1 of a router. What would be the addresses and masks?

       

       

      172.168.0.0

       

       

      172.168.32.0

       

       

      172.168.64.0

       

       

      172.168.96.0

       

       

      172.168.128.0

       

       

      172.168.168.0

       

       

      172.168.192.0

       

       

      172.168.224.0

       

       

      In Wendell Odom's Cert Guide he says to use the zero subnet unless class-full subnetting protocols are used (RIP 1) or the no IP subnet zero command has been issued. Therefore I would answer this question as:

       

       

      Fa0/0: 172.168.64.1 255.255.224.0

       

       

      Fa0/1: 172.168.96.1 255.255.224.0

       

       

      But is this correct? Should I count the zero subnet as a usable subnet?

       

       

      Any help much appreciated. Apologies if this has been discussed before.

       

       

        • 1. Re: SLSM: When to use the zero subnet?
          Paul Stewart  -  CCIE Security

           

          I think your answer is correct.

           

           

          172.168.0.0

           

           

          172.168.0.0 /19 (255.255.255.224) > First subnet

           

           

          172.168.32.0/19 > Second Subnet

           

           

          172.168.64.0/19 > Third Subnet

           

           

          172.168.96.0/19 > Fourth Subnet

           

           

           

           

           

          So the .1 ip address on the third and fourth subnet would be the answer to the question as posed.

           

           

          • 2. Re: SLSM: When to use the zero subnet?
            chris0161

             

             

             

             

            Thanks Paul

             

             

            So as a rule if I were asked a question like the one above I should always include the zero subnet unless there was a specific reason not to. (eg RIP 1 was in use).

             

             

            • 3. Re: SLSM: When to use the zero subnet?
              Paul Stewart  -  CCIE Security

              I would call the zero subnet, the first subnet. Sometimes in when counting 0 is the first number.

              • 4. Re: SLSM: When to use the zero subnet?
                Paul Stewart  -  CCIE Security

                Let me go one step further. Either way the subnets are numbered the same. In other words, the only way you would count them differently is if they said something to the effect of "third and fourth 'useable' subnet" alongside some other reason to make you believe that subnet-zero was not permitted.

                 

                 

                 

                 

                 

                 

                 

                So with the wording of the original question it would not matter. Even if it stated "no ip subnet-zero", 172.168.0.0/19 is still the first subnet. It just isn't useable. So you still have the same 3rd and 4th subnet. The only way this would be otherwise would be with the "no ip subnet-zero" in conjunction with the verbiage "useable addresses".

                 

                 

                 

                 

                 

                 

                 

                 

                 

                I would even have to think about if RIPv1 is mentioned. I'm not sure that would trigger the "no ip subnet-zero" behavior. There is nothing with the packet format that would keep it from working 0's and 1's subnet. There may be some trivial issue as to why it would trigger the behavior though.

                 

                Message was edited by: Paul Stewart

                Below is a screenshot of RIPv1 Updates on the 0 network. So I don't think there would be any reason that RIPv1 would not work with ip subnet-zero. Of course, the subnets must not be dis contiguous.

                • 5. Re: SLSM: When to use the zero subnet?
                  chris0161

                  I see so the zero subnet is always the first subnet but it is not always the first usable subnet.

                   

                  Below I have written two example questions relating to the subnet selection where the inclusion of the words “usable subnet” and “no IP subnet zero command” change the answer.

                   

                   

                  Example Question 1

                  Map the first IP address of the first usable subnet of network 172.168.0.0/19 onto FA0/0 when the command no IP subnet zero has been issued.

                   

                  Answer:

                  Fa0/0 172.168.32.1 Mask 255.255.224.0

                   

                   

                   

                  Example Question 2

                  Map the first IP address of the first subnet of network 172.168.0.0/19 onto FA0/0.

                   

                  Answer:

                  FA0/0 172.168.0.1 Mask 255.255.224.0