Subnetting Problem Contd

Discussion in 'MCSE' started by Myrt Webb, Jul 15, 2003.

  1. Myrt Webb

    Myrt Webb Guest

    Two days ago I posted the follwing question from the MS
    Trng Kit for 70-216. I received several responses but none
    that really explained why the answer to the question was
    selection 'd.'

    So, I lined out in binary form all of the eight IP
    addresses. It turns out that the 'd' answer that changes
    the subnet mask to 0/21 isolates the right most 3 bits of
    the 3rd octet.

    With the 0/21 mask there are two nets using 24 and 30 in
    the second octet. Then the host addresses are changed by
    one in the third octet.

    But, the question remains, why does this make the routing
    tables simpler for the router?

    Interesting problem, don't you love that MS does not give
    an explanation.

    "You are the new administrator of a 2000 node network.
    There is only one router on the entire network, which
    provides all the computers with Internet access. The
    company's ISP has assigned the following 8 network
    addresses to them:

    10.24.32.0/24
    10.24.33.0/24
    10.24.34.0/24
    10.30.35.0/24
    10.30.36.0/24
    10.30.37.0/24
    10.30.38.0/24
    10.30.39.9/24

    What subnet mask could you use to minimize the complexity
    of the routing tables while maintaining the existing
    Internet connectivity?

    a. 255.255.252.0
    b. 255.255.255.252
    c. 255.255.255.248
    d. 255.255.248.0 "
     
    Myrt Webb, Jul 15, 2003
    #1
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  2. Myrt Webb

    Manuel Guest

    Myrt,
    I think is because this way all the internal PCs can be on
    just one network with the same gateway ip address,

    for the router is easy , you define 2 lines only:
    10.24.32.0/21 my internal network
    all the rest go out

    for the local PCs or servers are:
    10.24.32.0/21 network is local to me
    all the other destinations send to the router.

    IF not, the router has to know where to direct 8 internal
    networks with at least 8 lines and probably assign 8 IP
    addresess to the internal network card or assign other
    devices as routers to help with it.

    Regards,
     
    Manuel, Jul 15, 2003
    #2
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  3. Myrt Webb

    Manuel Guest

    Myrt,
    I think is because this way all the internal PCs can be on
    just one network with the same gateway ip address,

    for the router is easy , you define 2 lines only:
    10.24.32.0/21 my internal network
    all the rest go out

    for the local PCs or servers are:
    10.24.32.0/21 network is local to me
    all the other destinations send to the router.

    IF not, the router has to know where to direct 8 internal
    networks with at least 8 lines and probably assign 8 IP
    addresess to the internal network card or assign other
    devices as routers to help with it.

    Regards,
     
    Manuel, Jul 15, 2003
    #3
  4. Myrt Webb

    Myrt Webb Guest

    I agree.

    With the original 0/24 subnet mask you have 8 separate
    nets with one host each. By changing the subnet mask to
    0/21 you have two nets with 3 hosts on the 10.24 net and 5
    hosts on the 10.30 net.

    I do not know a lot about routers but I imagine the two
    net configuration is better than the 8 net configuration.

    Thanks for your comments.
     
    Myrt Webb, Jul 16, 2003
    #4
  5. Myrt Webb

    freak Guest

    freak, Jul 16, 2003
    #5
  6. Myrt Webb

    Marko Guest

    I posted this reply:

    Jeffrey Woods has provided the best answer so far.

    Your ISP may not be a total idiot for "assigning" this IP
    space in the first place. It would only work on your
    10.24or30.y.z network and possibly to reach the ISP's
    servers / routers and that is likely it. However, if the
    ISP provides mail and proxy servers (possibly other
    services?) then this IP assignment is OK.

    10.24.x.y to 10.30.a.b would require a mask of 255.248.0.0
    to cover all those IP ranges mentioned.

    The 10.24.x.y ranges could be covered by "route add
    10.24.32.0 mask 255.255.253.0 (gateway IP here)"

    .....Assuming 10.30.39.9/24 is a typo and should be
    10.30.39.0/24 (cause the first is definitely wrong and
    does not exist)...

    Then route add 10.30.35.0 mask 255.255.251.0 (gateway IP)

    So far, neither route table addition looks like our answer.


    HOWEVER, if you concede that it is highly probable that
    all the class C subnets (those finishing with /24) start
    with either 10.24 OR 10.30, then 10.x.32.0 with a mask of
    255.255.248.0 will cover all IPs in the range 10.x.32.0 to
    10.x.39.255. Making "d" the correct choice....


    It it says d is right, then I would suggest the above
    paragraph is the solution.

    Anyone else care to have a go?

    .....care to kudos?

    .....care to flame?

    Marko Cosic
     
    Marko, Jul 16, 2003
    #6
  7. Myrt Webb

    Marko Guest

    I posted this reply:

    Jeffrey Woods has provided the best answer so far.

    Your ISP may not be a total idiot for "assigning" this IP
    space in the first place. It would only work on your
    10.24or30.y.z network and possibly to reach the ISP's
    servers / routers and that is likely it. However, if the
    ISP provides mail and proxy servers (possibly other
    services?) then this IP assignment is OK.

    10.24.x.y to 10.30.a.b would require a mask of 255.248.0.0
    to cover all those IP ranges mentioned.

    The 10.24.x.y ranges could be covered by "route add
    10.24.32.0 mask 255.255.253.0 (gateway IP here)"

    .....Assuming 10.30.39.9/24 is a typo and should be
    10.30.39.0/24 (cause the first is definitely wrong and
    does not exist)...

    Then route add 10.30.35.0 mask 255.255.251.0 (gateway IP)

    So far, neither route table addition looks like our answer.


    HOWEVER, if you concede that it is highly probable that
    all the class C subnets (those finishing with /24) start
    with either 10.24 OR 10.30, then 10.x.32.0 with a mask of
    255.255.248.0 will cover all IPs in the range 10.x.32.0 to
    10.x.39.255. Making "d" the correct choice....


    It it says d is right, then I would suggest the above
    paragraph is the solution.

    Anyone else care to have a go?

    .....care to kudos?

    .....care to flame?

    Marko Cosic
     
    Marko, Jul 16, 2003
    #7
  8. Myrt Webb

    Myrt Webb Guest

    I think the "point" of the question is that you can change
    a subnet mask to reduce the number of nets and thereby
    make the routings simpler.
     
    Myrt Webb, Jul 16, 2003
    #8
  9. Myrt Webb

    MikeF Guest

    Right, you got it. This question is made difficult only by MS
    inability to understand the meanings of words. And I am not kidding.

    The question is simply: you don't want to use all these networks. You
    want one network that will hold all your hosts. How do you do it?
    Ans, Supernetting, i.e., borrowing network id bits to use for host
    bits, increasing the number of available hosts on one network, instead
    of borrowing host bits to increase the number of separate networks
    which, of course, are called subnets.

    So the subnet in the answer is simply the one that gives you the range
    of addresses to hold 2000 or more hosts.

    Mike
     
    MikeF, Jul 16, 2003
    #9
  10. Myrt Webb

    Myrt Webb Guest

    Thanks for the confirming response.

    "Supernetting" or the opposite of subnetting is not
    something I have run into before.

    Live and learn.
     
    Myrt Webb, Jul 17, 2003
    #10
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