What exactly is Variable Length Subnet Masking
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Thread: What exactly is Variable Length Subnet Masking

  1. #1
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    What exactly is Variable Length Subnet Masking

    I've read up on this and im' supposed to give a definition on what this basically is, can someone give a basic/general definition of this protocol to me, and the basic setup of the protocol? Thanks in advance!

  2. #2
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    Read those:
    http://www.inetdaemon.com/tutorials/...t_masking.html
    http://www.tcpipguide.com/free/t_IPV...askingVLSM.htm <--- this is very good
    http://searchnetworking.techtarget.c...873628,00.html

    BTW, they are the on the first page of a google search

    After reading those, please post your doubts
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  3. #3
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    From all of this and other articles i've read i only gather that this method of subnet masking is used to conserve space for smaller type businesses so they wouldn't spend too much on a Class C because vlsm is taking the use of classless ips? Am i on the right track?

  4. #4
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    From the 2nd link, im wracking my brain trying to figure out what this exactly means:

    "The total number of hosts needed is thus 196. Without subnetting, we have enough hosts in our Class C network to handle them all. However, when we try to subnet, we have a big problem. In order to have six subnets we need to use 3 bits for the subnet ID. This leaves only 5 bits for the host ID, which means every subnet has the identical capacity of 30 hosts, as shown in Figure 70. This is enough for the smaller subnets but not enough for the larger ones. The only solution with conventional subnetting, other than shuffling the physical subnets, is to get another Class C block for the two big subnets and use the original for the four small ones. But this is expensive, and means wasting hundreds of IP addresses!"

  5. #5
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    I may be wrong here so any corrections would be appreciated because I have only really ever used CIDR, (read VLSM)...

    Pre-CIDR you could subnet down a network, (192.168.1.0), into a fixed number of subnets, (1, 2 ,4 8 etc). With CIDR you can break it down more efficiently, (1, 2, 3, 4, 5 etc). I believe that this is because under the old system the mask was applied across the entire subnet, (ie. if you wanted a subnet of 128 hosts you _had_ to have two subnets of 128 hosts), but under CIDR I can say I want a subnet of 128 hosts and two others of 64 each......

    That's my understanding..... Any education wil be very well taken.
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  6. #6
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    [i] I believe that this is because under the old system the mask was applied across the entire subnet, (ie. if you wanted a subnet of 128 hosts you _had_ to have two subnets of 128 hosts), but under CIDR I can say I want a subnet of 128 hosts and two others of 64 each......
    Best explanation of VLSM, TS. Its just like that.

    Conventional subnetting --> just one mask for entire network
    VLSM--> you can have subnets with different sizes, like 64, 128 32 on the SAME network.

    Why VLSM is so important to know? because there are some routing protocols that DONT allow VLSM, like RIP (although i think that there is a RIP version that can, can someone confirm that?).
    Meu sítio

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  7. #7
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    Subnetting used to be known as classes. You would have A, B en C classes. Firms used to be given a class, let's say "IBM" would have gotten a A-class subnet, which would give a default subnetmask like 255.0.0.0, which of course nowadays is utterly insane cause of the insane great network and hosts.
    A-class was a default subnet mask of 255.0.0.0, B-class 255.255.0.0, C-class 255.255.255.0. They used to decide which class you would get by how big your company was/is.

    VLSM is the possibility of dividing for.ex a C-class subnet (255.255.255.0) into different variable subnets.

    Example:

    You can divide this subnet in for.ex. an 255.255.255.128 subnet (128 hosts, actually 126 usable) and 2 255.255.255.192 subnets each consisting out of (64 - 2) = 62 usable hosts. There are of course lots of other combinations. This one is just to show you the meaning of vlsm.

    192.168.0.0/255.255.255.128 =128 - 2 (first and last) = 126 usable ip's
    192.168.0.128/255.255.255.192 = 64 - 2 (first and last) = 62 usable ip's
    192.168.0.192/255.255.255.192 = 64 - 2 (first and last) = 62 usable ip's

    This would divide that c-class in 3 different subnets giving a better usage, and making the networks fit your needs, since also the demand for ip's increased alot the last decade and if the old way of subnetting would be holded we would have run out of ip's nowadays. Therefor the vlsm and ofcourse the coming of ipv6 which will replace the ipv4. The vlsm its purpose actually was/is a better administration of public ip adresses cause int the ipv4 the free ranges are getting scarse.

    Cacosapo, RIP_v2 should be able to handle vlsm, as also others like ospf and eigrp.

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