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  1. Very clear and easy to understand thank you.

  2. Crystal clear. You have explained it very nicely.

  3. Hi Rene ,

    Is “Discontiguous Network” issue is due mainly classfull routing lookup ? , because it will do look up only classfull manner , like router has to do a look up for destination network /24 , as the 1st octate is belongs to class A range it wont check further the the mask part , if network not existing it will drop it ?

    Can you guide on “Discontiguous Network” and its solution aprroach .


  4. Hello Sameer

    For the explanation, I will use the subnets used in the lesson.

    The problem with classful networks is that if there is a destination of this will be installed in the routing table as since 172.16.X.X is a class B network. This works fine as long as all destinations of are to be routed via the same interface.

    However, if you have a case such as this, discontiguous networks with classful routing presents a problem:
    Assuming a classful routing protocol is being used, R3 will inform R2 that it has a destination to and R2 will put the following into its routing table:

    to reach, route via Fa0/1 with a next hop IP of

    But R1 also sends a routing update to R2 stating that it has a route to so R2 will put the following into its routing table:

    to reach, route via Fa0/0 with a next hop IP of

    During the next routing updates, R3 and R1 will alternately inform R2 that they have routes to these destinations, while all the time R2 is installing the classful network into the routing table, causing incorrect routing to occur.

    So, this is why discontiguous networks can cause problems with classful routing.

    In order to solve such issues, there are two solutions:

    1. The first involves adjusting your addressing scheme so that networks are not discontiguous, that is, all class A networks can be reached via a single interface, and similarly for class B and class C. This is not always possible and can be somewhat restrictive.
    2. The ideal solution, and the one that is considered best practice, is to use a classless routing protocol so that you can define specific subnets within classful networks and route in a classless manner.

    The second choice should always be the selected solution in all modern IP networks.

    I hope this has been helpful!


  5. Thank you Laz!!

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