*REAL QUESTION* - 70-291: VLSM's

Discussion in 'MCSA' started by Matrixx333, Apr 30, 2009.

  1. Matrixx333

    Matrixx333 Guest

    Hey guys,

    I'm studying for the 70-291 and I'm having a hard time understanding
    VLSM's. I think I have it now, but I just need someone to confirm my
    understanding. Below is an example Class B default network that I
    broke up using my understanding of VLSM. If someone could confirm if
    it is correct, and if not, point out my mistakes and help to explain
    where I went wrong, I would greatly appreciate it. Thanks!

    Default Network
    150.1.0.0/16 - original network

    150.1.0.0/24 (255.255.255.0) makes 256 subnets with 256 hosts per
    subnet

    150.1.1.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    150.1.2.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    150.1.3.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    150.1.4.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    .....
    .....
    .....
    150.1.56.0/30 (255.255.255.252) - 64 subnets with 4 addresses per
    subnet (2 assignable, 1 net id, 1 broadcast)
    150.1.56.4/30
    150.1.56.8/30
    150.1.56.12/30
    150.1.56.16/30
    ....
    ....
    ....
    150.1.56.248/30
    150.1.56.252/30

    150.1.57.0/30 (255.255.255.252) - 64 subnets with 4 addresses per
    subnet (2 assignable, 1 net id, 1 broadcast)
    150.1.57.4/30
    150.1.57.8/30
    150.1.57.12/30
    150.1.57.16/30
    ....
    ....
    ....
    150.1.57.248/30
    150.1.57.252/30

    150.1.58.0/30 (255.255.255.252) - 64 subnets with 4 addresses per
    subnet (2 assignable, 1 net id, 1 broadcast)
    150.1.58.4/30
    150.1.58.8/30
    150.1.58.12/30
    150.1.58.16/30
    ....
    ....
    ....
    150.1.58.248/30
    150.1.58.252/30

    150.1.59.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    150.1.60.0/24 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    .....
    .....
    .....
    150.1.128.0/27 (255.255.255.224) - 8 subnets with 32 addresses per
    subnet (30 assignable, 1 net id, 1 broadcast)
    150.1.128.32/27
    150.1.128.64/27
    150.1.128.128/27
    150.1.128.160/27
    150.1.128.192/27
    150.1.128.224/27
    .....
    .....
    .....
    150.1.254.0 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
    150.1.255.0 (1 subnet, 254 hosts, 1 net id, 1 broadcast)
     
    Matrixx333, Apr 30, 2009
    #1
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  2. Variable Length Subnet Masks... yes... as if subnetting weren't complex
    enough, then they came along and created this ability to break up a network
    into multiple subnets with different subnet masks. :)
    Your example, while perhaps unnecessarily complex, does appear to correctly
    capture the essence of VLSM.

    Let me offer a simpler example, just to confirm the scenario, and in the
    form you're likely to see the concept tested:

    Imagine a single Class C network -- 192.168.1.0 -- with 172 computers.

    We need six subnetworks, two with 50 computers, two with 30 computers, and
    two with 6 computers.

    Let's first set up the two 50-computer subnets. For this we'll need two /26
    subnets (netmask 255.255.255.192). We'll use the following:
    192.168.1.64/26 (192.168.1.65 - 192.168.1.126)
    192.168.1.128/26 (192.168.1.129 - 192.168.1.190)

    Then the two 30 computer subnets. For this we'll need two /27 subnets
    (netmask 255.255.255.224). We'll use the following:
    192.168.1.32/27 (192.168.1.33 - 192.168.1.62)
    192.168.1.192/27 (192.168.1.193 - 192.168.1.222)

    Finally, the two 6-computer subnets. For this we'll need two /29 subnets
    (netmask 255.255.255.248). We'll use the following:
    192.168.1.24/29 (192.168.1.25 - 192.168.1.30)
    192.168.1.224/29 (192.168.1.225 - 192.168.1.230)

    At this point we could also have used several other /29 subnets, including:
    192.168.1.8/29
    192.168.1.16/29
    192.168.1.232/29
    192.168.1.240/29

    Notice also, although no longer strictly required, I've avoided the use of
    the first and last subnet in the range (e.g. the subnet with network ID '0'
    and the subnet with broadcast ID '255').

    The end results of our configuration:

    Net1: 192.168.1.24/29 (6 hosts)
    Net2: 192.168.1.32/27 (30 hosts)
    Net3: 192.168.1.64/26 (50 hosts; with 12 unused HostIDs)
    Net4: 192.168.1.128/26 (50 hosts; with 12 unused HostIDs)
    Net5: 192.168.1.192/27 (30 hosts)
    Net6: 192.168.1.224/29 (6 hosts)

    The key point being here is that we've divided up a specified network into
    multiple subnets with a *variable* length subnet mask (/26, /27, or /29),
    depending on the specific subnet.

    Hope this helps.

    --
    Lawrence Garvin, M.S., MCITP:EA, MCDBA
    Principal/CTO, Onsite Technology Solutions, Houston, Texas
    Microsoft MVP - Software Distribution (2005-2009)

    MS WSUS Website: http://www.microsoft.com/wsus
    My Websites: http://www.onsitechsolutions.com;
    http://wsusinfo.onsitechsolutions.com
    My MVP Profile: http://mvp.support.microsoft.com/profile/Lawrence.Garvin
     
    Lawrence Garvin [MVP], Apr 30, 2009
    #2
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  3. Matrixx333

    Matrixx333 Guest

    The end results of our configuration:
    Thank you for the time to reply...It has helped to solidify my
    understanding and give me the confidence that I needed. So the ranges:

    192.168.1.1 - 192.168.1.23 and
    192.168.1.225 - 192.168.1.254

    basically go unused until there is a need for them at which point they
    themselves can be given a VLSM?

    Thanks again!
     
    Matrixx333, May 1, 2009
    #3
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