4 Replies Latest reply: May 15, 2009 7:24 AM by Jared RSS

    Configure mask /29 and /30 on a router


      Hi all

      I spent an hour playing with the CCNA rack at mindtech.com and wasnt able to configure interfaces the way I wanted them.

      The routers are running IOS 12.2

      My question is : Can you configure a router to have /30 for its Serial0 and /29 for its Ethernet0?

      I was not able to do that last night even after making sure ip classless had been enabled. What's wrong?

        • 1. Re: Configure mask /29 and /30 on a router

          ok, so if you assign an interface an IP of /30, I would assume that .4 is your network, .5 and .6 are your hosts and .7 is your broadcast.  Then a new network of where .8 is the network address, .9 is a host, which happens to be the interface.  .10-.14 are also hosts, which would give you a total of 6 hosts and .15 would be the broadcast.


          That seems like it should work... if the router knows it is allowed to use the zero subnet, which would be, which would have .0 as the network, .1 and .2 as hosts and .3 as the broadcast.  Otherwise, it may think there is an overlap and not allow your addressing.


          was ip subnet-zero also configured on your device?

          • 2. Re: Configure mask /29 and /30 on a router

            Yes, both ip subnet-zero and ip classless are enabled.


            I could have made life simpler I guess by going for /29 for Eth0 and /30 for Ser0.


            But I wanted to test and see it for myself to see if routers can be configured with a 192.168.1. network with only the mask separating/distinguishing the last octet.


            I even tried it with /29 for E0 and /30 for S0, but no luck.  Maybe there is something wrong with mindtechcom's routers or IOS.  The router says that the E0 address overlaps with the S0 address.

            • 3. Re: Configure mask /29 and /30 on a router

              Hi Sulley,

              You might be confused about the purpose of the mask, because you thought that it would be possible to give the same IP adress to multiple interfaces but with different masks.

              Actually it's not possible, because the mask in itself is not an identifier for the interface, it's the IP address that identifies uniquely each interface. The mask main purpose is to identify which part on the IP address is the network part, and which is the host part. This lets the interface know which subnet it belongs to.


              Regarding your initial question, I agree with Jared about the subnet ranges, but I don't see why the "ip sunet-zero" would have an impact. I don't think so, because your subnets are:


     /30 (which /30 belongs to).

     /29  (which /29 belongs to).


              I don't know what's wrong with your config,

              • 4. Re: Configure mask /29 and /30 on a router



                I see that you have marked your question as answered.  Hopefully it was just a fat finger in your config or the fact that you tried to have the same IP on two interfaces.


                I just set up your original explained config with on one interface and on another interface in my test lab and had no issues.



                What did you find to resolve the problem?