Subnetting Nighmare
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Thread: Subnetting Nighmare

  1. #1
    AO's Mr Grumpy
    Join Date
    Apr 2003
    Posts
    903

    Subnetting Nighmare

    There are some thing's in life that are quite diffiult to truly understand, and when you finally get it , it's like a revelation, and usually never forgotton. For me my present nightmare is subnetting. Yes I've had the lecture's, read the book's, tried all the method's, and before you ask "yes--of course I've searched Google. But what I was looking for was a very, very, very simple method of calculation "No--not with an IP calculator"I have come across a site Http://www.cscug.org/Presentations/S..._Your_Head.pdf which claims to enable you to "Subnet in you head in 90 Minutes" Ha Ha. I don't think so, but who knows.I would be interested to hear of any simple method used with success by AO member's
    Computer says no
    (Carol Beer)

  2. #2
    Senior Member
    Join Date
    Jan 2003
    Posts
    3,914
    Well with subnetting, the key (as far as I found) is to get a class C address down pat before you even attempt A & B (which use the same method).

    How your only SM (subnet mask) possibilities for a Class C are 252 (6 bits), 248 (5 Bits), 240 (4 bits), 224 (3 bits), 192 (2 Bits) because you can only borrow 2 - 6 bits. It's useful to have those numbers memorized, or at least I think so.

    2^number of bits borrowed will give you your subnets
    2^bits left will give you your hosts / subnet

    If i borrow 4 bits.
    I'll have 2^4 = 16 subnets and I'll have 2^(8-4) = 2^4 = 16 hosts/subnet.
    Well I have my Class C possibilities memorized (or look above), so if I borrow 4 bits my SM is 255.255.255.240. Say my network address was 204.17.16.0.
    My first subnet would start @ 204.17.16.0 and go to 204.17.16.15, since it contains my original network address it is an unusable subnet.
    The Next subnet would contain 16 hosts (determined above) and start with 204.17.16.16.... to get 16 hosts it would have to end at 204.17.16.31. This subnet is useable. 204.17.16.16 would be the Subnet Network Address and 204.17.16.31 would be the Subnet Broadcast address. We'd then keep counting up by 16s until we go to the last grouping which would be
    204.17.16.240 - 204.17.16.255 which would be unusable because it contains the original broadcast address....

    After you get class C subnetting down pat.. you can start working with the others (A&B)... I'll give ya a hand with them if you require it.. and if this was at all useful too ya.. throw me a PM if you need any help

    HT
    IT Blog: .:Computer Defense:.
    PnCHd (Pronounced Pinched): Acronym - Point 'n Click Hacked. As in: "That website was pinched" or "The skiddie pinched my computer because I forgot to patch".

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