2 Replies Latest reply: Jan 14, 2014 11:08 PM by Chuks RSS

    VLSM OVERLAP

    Chuks

      Hello everyone,

       

      can someone please help with this topic VLSM overlapping, just basic description and examples, what it is and how to solve it. I have read it but it still eludes me.

       

      Thanks

        • 1. Re: VLSM OVERLAP
          Alessandro

          HI Chucks,

           

          I'll try to explain you:

           

          VLSM Overlapping considers the situation in which some ip addresses that are

          in a subnet, are also in another subnet, so the subnets overlaps and it should

          be avoided.

          For example consider the following two subnets: 10.8.100.96.0/20 and 10.8.104.0/24

           

          The first subnet has this usable ip addresses range: 10.8.96.1 to 10.8.111.254.

          The second subnet has this usable ip addresses range:10.8.104.1 to 10.8.104.254

           

          The two subnet overlaps because, for example, in the first subnet can be a host

          with the ip address 10.8.104.10, but also in the second subnet can be a

          host with the same address.

           

          When you have a problem that give you some subnets (or ip addresses from which calculate subnets) and ask you to find

          which ones overlaps, you have to:

          order the subnets from the lowest to the highest;

          find the respective subnets broadcast address;

          compare if the usable ip addresses of a subnet are also in another subnet (overlaps).

          An example (taken from the Official Cert Guide):

          172.20.1.237/30

          172.20.1.222/27

          172.20.1.225/28

           

          These are three ip addresses.

          From the lowest to the highest:

          172.20.1.222/27   subnet address: 172.20.1.192 subnet broadcast address:172.20.1.223

          172.20.1.225/28   subnet address: 172.20.1.224 subnet broadcast address:172.20.1.239

          172.20.1.237/30   subnet address: 172.20.1.236 subnet broadcast address:172.20.1.239

           

          The first subnet 172.20.1.192 doesn't overlaps the second;because they are in order

          it won't overlaps the third.

          The second subnet 172.20.1.224 overlaps with the third 172.20.1.236, because a part of the range

          of addresses in the second subnet (172.20.1.236 to 172.20.10.238) are also in the third

          subnet.

          So, the second and the third subnet overlap.

           

          I hope I've given you a clear answer.

          • 2. Re: VLSM OVERLAP
            Chuks

            Hello Alessandro,

             

            Thanks now i understand.....much appreciated.