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Thread: Subnetting Questions

  1. #1
    Join Date
    Oct 2003

    Subnetting Questions


    I have a couple of questions regarding subnetting.

    Lets say i have an IP address
    Subnetting mask

    Anyway my questions are how do i work out the following ? Without a Subnet calculator

    1. subnet address for the given host IP address
    2.Range host for the addresses for this subnet
    3.broadcast address for this subnet
    4.total number of useable subnets
    5.number of valid host IP addresses per subnet
    6.Subnetwork address with slash(/) mas for the assigned subnet
    7.Default gateway (first useable subnet address)


  2. #2
    Senior Member
    Join Date
    Mar 2005


    First, this question has mistakes, under classful addressing schemes.
    If from a noob it's understandable, but if from someone with a faint knowledge of IP standards, it's a setup (a bait).

    But I will take the bait, either way.

    Some problems are:
    1) You give a Class A (/8) address but couple it with a Class C (/24) type subnet mask. Not in accordance with RFC 950 guidelines, you are introducing confusion into a private network with an Internet routable address.
    2) Since you didn't start with a proper IP and subnet mask combination, your example does not get a proper CDIR slash, although a /24 would probably be the closest (which is
    3) Following the RFC standards way of matching IP addresses with proper subnet masks is best, using a subnet calculator can prevent mistakes keeping you from odd combinations (as this example is).
    4) In calculating your scenario, I came up with weird combinations and potential problems with my answers. In my opinion, it's best to only pick the choices an IP calculator would give you.
    5) If you were to key your example into a classful router and/or computer I'm really not sure what would happen. Probably a CDIR aware router would overlook your choices and give you a setup based on a subnet mask of

    Anyways, as for calculating various IP addresses, it's easier than it looks.
    The easiest way to do it is to convert the ip address and the subnet mask to binary representation.

    First take with subnet mask and convert it to binary:
    01111011.11101001.00000100.00100010 <---IP Address (
    11111111.11111111.11111111.00000100 <---Subnet Mask (

    If you do a binary AND between these two numbers, you get the Network address:
    01111011.11101001.00000100.00100010 <---IP Address (
    11111111.11111111.11111111.00000100 <---Subnet Mask (
    -----------------Binary AND-----------------------
    01111011.11101001.00000100.00000000 <---Network Address
    which translated back to dotted decimal notation is <----Network Address
    which shows as first usable IP on this network

    To get the host portion, invert the subnet mask and perform a binary AND with the ip address:
    01111011.11101001.00000100.00100010 <---IP Address (
    00000000.00000000.00000000.11111011 <---Inverted Subnet Mask
    -----------------Binary AND-----------------------
    00000000.00000000.00000000.00100010 <---Host portion
    which translated back to dotted decimal notation is

    To obtain the broadcast address, we perform a binary function called XOR:
    Take the inverted subnet mask and perform a binary XOR with the network address:
    01111011.11101001.00000100.00000000 <---Network Address
    00000000.00000000.00000000.11111011 <---Inverted Subnet Mask
    -----------------Binary XOR-----------------------
    01111011.11101001.00000100.11111011 <---Broadcast Address
    which translated back to dotted decimal notation is

    Simple math calculates the total number of assignable hosts minus 123.233.251 equals 250 hosts. (don't count the network address or the broadcast address)

    Your subnet mask ( doesn't leave but one valid subnet. (Although in actuality I came up with numerous subnets based on this combo, but I'd rather not discuss it here. This is one reason why I think this is a question (bait) from a classroom discussion.)

    Classful IP addressing references:
    Overall RFC Index
    RFC 1918 - Address Allocation for Private Internets
    RFC 1219 - On the Assignment of Subnet Numbers
    RFC 950 - Internet standard subnetting procedure
    RFC 940 - Toward an Internet standard scheme for subnetting
    RFC 932 - Subnetwork addressing scheme
    RFC 917 - Internet subnets

    (editing: typos and formatting, you didn't think this was easy formatting did you?)

    Beta tester of "0"s and "1"s"

  3. #3
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