Subnetting Help

Posted on 2011-04-28
Last Modified: 2012-05-11
I am currently reading/learning about subnetting I have mostly got the hang of it now but am struggling with
a couple of questions on the practice exam.

what is the valid ip range for the subnet

what is the valid ip range for the subnet

the number of valid subnets on the is ?

please could someone explain the process to work out the above I'm fine with anything /24 or above but
stuggling with these.

Many Thanks
Question by:Ross-C
    LVL 1

    Accepted Solution


    Subnet mask:
    Valid Host IP Range -

    Subnet mask:
    Valid Host IP Range -  subnet
    Valid Host IP Range -

    The rational behind the slash notation "/" is the number of bits of the binary representation of the IP address are used to determine the subnet address.
    "/24" is easy because it uses all of the first three octets to determine the subnet leaving you with just the last octet for hosts, x.x.x.1 - x.x.x.254

    i.e. = 11111111.11111111.11111111.11111111
    so a "/15" subnet means that the first 15 bits are the network address and the rest is used for determining host addresses.
    so for the example:
    "00010100.0000000"0.00000000.00000000 = the quoted portion is the bits used for the subnet.
    So the valid host IP range will be from 0.00000000.00000000  to 1.11111111.11111111
    convert from binary and you get x.0.0.1 to x.1.255.254 (excluding the all 0s and all 1s as those addresses are used for the network address and broadcast address)
    The first 15 bits being the subnet gives you the subnet mask:11111111.11111110.00000000.00000000 =

    This is a really abbreviated explanation and relies on an understanding of how to count in binary, or at least convert from binary to decimal and back again.

    Hope this helps. You can do a search online for Subnet calculators that will take the work out of calculating the subnet masks and valid hosts for a giving ip/#
    LVL 15

    Assisted Solution

    I get slightly different answers for the first:

    what is the valid ip range for the subnet -

    and third:
    the number of valid subnets on the is ?
    254, or if it's old school which doesn't allow an all zero subnet, 253

    @Ross-C, for the above I assume you understand the underlying binary and just want an easy way to work it out?
    LVL 11

    Author Comment

    Yes that's right Frabble. I have been studying submitting and understand the binary etc. I am just struggling with the best way to work out questions like the ones in my post.
    LVL 1

    Expert Comment

    Thanks for the correction Frabble. That was a typo on my part for the address range.
    LVL 11

    Author Closing Comment

    Many thanks to you both

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