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subnetting question

Discussion in 'Routing & Switching' started by azrael2000, Sep 11, 2008.

  1. azrael2000

    azrael2000 New Member

    Hi All.

    Have a little question about subnetting.

    I understand how to subnet an address to get the number of hosts or networks required.

    That is not a problem; however, if I have subnetted an IP, then want to subnet one of the subnets (hope that makes sense) how do I do it?

    I've tried a couple of different books but nothing is making sense...

  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    Just take the subnet range and run it through again? Unless there are other constraints I would have thought this would be the way to do it.

    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. azrael2000

    azrael2000 New Member

    Ok, so if I take, for example and make it a /28... that would leave me with subnets like,, and so on...

    If I want to take the and make it into a /30, then what would the new subnets be and why?

    That's where I get a little confused.

  4. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

    Just subnet it into /30-sized chunks. How many addresses are within a /30 subnet? Four, right? Two host addresses, a network address, and a broadcast address. So, split up a /28-sized chunk (of 16 addresses) into multiple /30-sized chunks:
    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+
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  5. gamava

    gamava New Member

    Try Variable length subnet mask(VLSM)
  6. albertc30

    albertc30 Kilobyte Poster

    Any more doubts mate?
    Need any more help?

    Lets just hope that I can make it easier for you.

    Class C IP address =
    IP addresses range from to
    Network ID = Broadcast IP
    Gateway = (default class C subnetmask)

    Lets subnet it to a

    Network IP ---------------- Range -------------- Broadcast IP - / - - / - - / - - / -

    So by subnetting it using 2 bits from the host portion of the address you got 4 networks and 62 available IPs peer network.

    Lets say that you used the first range and now you need to ad another network to the existing network and you need just 28 hosts in that new network.

    How many bits do we need to take from the host portion of the address to allow at least 28 hosts?
    Not 1 as the first bit gets you 126 hosts peer network, not the first 2 as you'd get 62 hosts peer network but the first 3 as you'd get 30 available IPs peer network, more then enough for your new network of 28 hosts.
    One minor detail here that plenty of people seem to forget. Whatever many pcs are in a network they will almost certainly be conected to a router and that router's interface will also need an IP address, even worst if you are dealling with VLANS where the router's interface mitght need one IP address peer VLAN and all that need to be taken into account. So in this example if you'd be using 4 VLans having this network sub subneted to you wouldn't have enough available IPs to acomodate this lan, so you'd need it to be sub subneted to a

    Back in track, the next available and unsuable subnetwork is the so lets change it to a So we have; - / - - / -

    And that would be that subnetwork sub subneted into another subnetwork.

    Remember you can only sub subnet unused subnets and be aware not to overlapping.

    Hope it helps and not have complicated it to much.
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    WIP: 220-701 - A+

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