Subnetting Formula

Discussion in 'MCSA' started by John, Jun 30, 2003.

  1. John

    John Guest

    I just took the 215 and had no subneting questions. I
    don't think subnetting is weighed too heavy on this test
    >-----Original Message-----
    >Oh man! Y'all are making me feel very good about the 215
    >exam with all of this subnetting stuff! I just learned
    >this a few months ago in the Cisco Networking Academy.
    >After all the repitition on subnetting it's almost like
    >adding 2 and 2 now. Those formulas are very good to know.
    >I have a buttload of subnetting handouts that I have read
    >a million times.
    >
    >Aside from certification, I would download this free
    >Subnet Calculator that I have. I think it's from Solar
    >Winds. Don't know the web address for it so go to
    >download.com and just do a search for subnet calculator

    or
    >solar winds and see what you find. It will save you much
    >time in the real world.
    >
    >Don't forget about the Subnet address and Broadcast
    >address when using those formulas! I know Cisco loves to
    >pick your brain on that so Microsoft is probably just as
    >evil.
    >
    >
    >
    >>-----Original Message-----
    >>The below subnetting formula has saved me much heartache

    >on two exams
    >>now, so I'd thought I'd share it - never know, it might

    >come in handy
    >>one day.
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>FORMULA
    >>
    >>
    >>
    >> subnets hosts
    >>
    >>/x : 2^y , 2^z
    >>
    >>
    >>
    >>
    >>
    >>for CLASS A addresses ( 1-126.X.X.X)
    >>
    >> x+z=32 and y+z=24
    >>
    >>
    >>
    >>for CLASS B addresses (128-191.X.X.X)
    >>
    >> x+z=32 and y+z=16
    >>
    >>
    >>
    >>for CLASS C addresses (192-223.X.X.X)
    >>
    >> x+z=32 and y+z=8
    >>
    >>
    >>
    >>
    >>
    >>(hint-- notice how x+z=32 for any class of addresses,
    >>
    >> all you have to remember the y+z=? per class)
    >>
    >>
    >>
    >>REMEMBER TO ALWAYS SUBRTACT 2 FROM THE HOST COLUMN
    >>
    >>(ie, all zeros host is reserved to indicate network ID
    >>
    >> all ones host is reserved to indicate a broadcast)
    >>
    >>
    >>
    >>
    >>
    >>Examples:
    >>
    >>
    >>
    >>CLASS A EXAMPLE
    >>
    >>
    >>
    >>Your given a CLASS A /17 address,
    >>
    >>how many subnets and hosts ?
    >>
    >>
    >>
    >> subnets hosts
    >>
    >>/17: 2^9 , 2^15
    >>
    >>
    >>
    >># of subnets: 512
    >>
    >># of hosts: 32768-2=32766
    >>
    >>
    >>
    >>explanation:
    >>
    >>if you know x=17, then 17+z=32 or z=15
    >>
    >>if you know z=15, then y+15=24 or y=9
    >>
    >>
    >>
    >>
    >>
    >>CLASS B EXAMPLE
    >>
    >>
    >>
    >>Your given a CLASS B /23 address,
    >>
    >>how many subnets and hosts ?
    >>
    >>
    >>
    >> subnets hosts
    >>
    >>/23: 2^7 , 2^9
    >>
    >>
    >>
    >># of subnets: 128
    >>
    >># of hosts: 512-2=510
    >>
    >>
    >>
    >>explanation:
    >>
    >>if you know x=23, then 23+z=32 or z=9
    >>
    >>if you know z=9, then y+9=16 or y=7
    >>
    >>
    >>
    >>
    >>
    >>CLASS C EXAMPLE
    >>
    >>
    >>
    >>Your given a CLASS C /26 address,
    >>
    >>how many subnets and hosts ?
    >>
    >>
    >>
    >> subnets hosts
    >>
    >>/26: 2^2 , 2^6
    >>
    >>
    >>
    >># of subnets: 4
    >>
    >># of hosts: 64-2=62
    >>
    >>
    >>
    >>explanation:
    >>
    >>if you know x=26, then 26+z=32 or z=6
    >>
    >>if you know z=6, then y+6=8 or y=2
    >>
    >>
    >>
    >>
    >>
    >>
    >>
    >>In all the examples above, you can also do the reverse

    >and solve for
    >>"x". This comes in handy if you need to determine the

    >subnet mask
    >>after being told how many subnets and hosts you require.
    >>.
    >>

    >.
    >
     
    John, Jun 30, 2003
    #1
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