#1
  1. No Profile Picture
    Contributing User
    Devshed Newbie (0 - 499 posts)

    Join Date
    Jan 2012
    Posts
    37
    Rep Power
    3

    Question about subnetting


    I am having trouble with this problem. I tried to work out some of it, but I am stuck on other parts of it.

    An organization is granted the block 16.0.0.0/8. The administer wants to create 500 fixed-length subnets.

    Determine the mask:
    32-8 = 24, so 24 rightmost bits are reserved for network. So the mask would be 255.0.0.0 right?

    Find the number of addresses in each subnet:
    2^24 = 16,777,216 addresses / 500 = 33,554 addresses in each subnet.

    Find the first and last address in subnet 1.
    This is where im stuck

    Find the first and last address in subnet 500.
    This is where im stuck


    Any help with this I would greatly appreciate, I am trying to prepare for an upcoming exam.

    Thank you!
  2. #2
  3. No Profile Picture
    Lost in code
    Devshed Supreme Being (6500+ posts)

    Join Date
    Dec 2004
    Posts
    8,301
    Rep Power
    7170
    32-8 = 24, so 24 rightmost bits are reserved for network.
    Actually that's backwards, the /8 means that 8 bits are reserved for the network and the remaining 24 are for the host.

    So the mask would be 255.0.0.0 right?
    If the entire block was a single subnet then yes, but in this case the network is being divided into 500 subnets. To represent 500 values you need 9 bits, because 2^9 = 512. So your subnet mask actually needs to cover the first 8+9 bits of the address.

    Code:
    16.0.0.0:  0001 0000  0000 0000  0000 0000  0000 0000
               nnnn nnnn  ssss ssss  shhh hhhh  hhhh hhhh
    So this leaves you with 15 bits for the host. The n is your fixed network part, the s is your internal network part and the h is your host part.

    So nnnn nnnn is fixed as 0001 0000. The ssss ssss s can vary from 0000 0000 0 to 1111 1111 1, and that defines the subnet number. The hhh hhhh hhhh hhhh part can vary from 000 0000 0000 0000 to 111 1111 1111 1111, and that defines the host part.

    When h is all 1's you have a broadcast address, which shouldn't be assigned, so I would assume that the last address that your question is looking for is actually h's = 111 1111 1111 1110.

    The part I can't quite remember is whether or not you need to reserve the address where h is all 0's.
    PHP FAQ

    Originally Posted by Spad
    Ah USB, the only rectangular connector where you have to make 3 attempts before you get it the right way around
  4. #3
  5. No Profile Picture
    Contributing User
    Devshed Newbie (0 - 499 posts)

    Join Date
    Jan 2012
    Posts
    37
    Rep Power
    3
    Thank you!

    I finally figured it out.

    The trick is just to work with numbers once you have converted them to binary, it makes it a whole lot easier.

    Thanks again for your help

IMN logo majestic logo threadwatch logo seochat tools logo