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Subnetting Question

Discussion in 'General Cisco Certifications' started by datarunner, Aug 20, 2008.

  1. datarunner

    datarunner Byte Poster

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    hi all

    i thought i was ok at subnetting til i got a question simillar to this:

    which addresses are valid host addresses given the subnet mask 255.255.255.248

    192.168.200.87
    194.10.10.84
    223.168.210.100
    200.100.100.154
    200.152.2.160
    196.123.142.190

    choose 3

    if someone could should me how to work this out i'd be grateful

    cheers
     
    Certifications: A+, N+, MCP 210, 270, HNC Networking
    WIP: MCSA
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Hm - I'd say that 4 of them are valid and 2 invalid.

    Method - use the mask to find the *machine address* (i.e. the part where the mask is zero). If the machine address is all zeros or all ones then it isn't a valid *host* address.

    Of course - I may not have spotted some nuance....


    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. MacAllan

    MacAllan Byte Poster

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    Quick method:

    256 - 248 gives 'magic number' of 8.

    Therefore every subnet network address will be a multiple of 8, starting with 0 :
    0, 8, 16, 24, 32....etc

    Every broadcast address will be one less than a possible subnet network address :
    7, 15, 23, 31......etc

    Any value in the fourth octet that matches one of these numbers can not be a host address.

    Easy really.... :)
     
    Certifications: A+, N+, CCNA
    WIP: CCNP, Linux+
  4. datarunner

    datarunner Byte Poster

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    hi buddy

    thanx, easy when u know how

    ok wot about this one:

    10.149.77.147 /12

    wot is the broadcast address

    10.255.255.255
    10.144.255.255
    10.149.255.255
    10.149.191.255
    10.159.255.255

    cheers
     
    Certifications: A+, N+, MCP 210, 270, HNC Networking
    WIP: MCSA
  5. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Why don't you give it a shot with the information you have rather than ask someone to give you the answer?
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  6. datarunner

    datarunner Byte Poster

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    not lookin for the answer buddy. just looking for a way to work it out. the /12 is confusing me.

    am i rite in saying that the magic number here is 16 so 0, 16,32,48,64

    after that i get lost

    cheers
     
    Certifications: A+, N+, MCP 210, 270, HNC Networking
    WIP: MCSA
  7. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    The /12 indicates how many '1' bits there are in the mask. (Counting from the 'left' - i.e. the most significant position).

    So calculate from there.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  8. datarunner

    datarunner Byte Poster

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    ah the magic number is 16 so the network will be 144, so the next network is 160 - 1 = 159

    answer 10.159.255.255
     
    Certifications: A+, N+, MCP 210, 270, HNC Networking
    WIP: MCSA
  9. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Just wanted to see how far you could work it out on your own, that's all. :) Then we can give you a nudge in the right direction, as Harry has graciously done. :)

    And... unless my calculations are off, you have answered correctly with 10.159.255.255. ;)
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
    WIP: Just about everything!
  10. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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