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Home > Archive > CCNP > May 2002 > Vlsm
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| lindolfoal 2002-05-01, 3:52 pm |
| Hi,
I am studying for the 640-503 and I did not obtain to understand right the concept on VLSM. Somebody can explain me better on what it is? So that he is used? When it is used?
Tanks. | |
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| VLSM used to take advantage of your assigned address to the limits. Here is the way I understand it (hope I understand it correctly), You subnet the address that has been assigned to you, let us say you have the following subnets, 16,32,48,64 ...etc, you used subnets 16,32, then you need to assign addresses to your WAN connection (each serial connection has to be on same subnet) so you take UNUSED subnets in this case subnet 48 and further subnet it, so instead of having one subnet(48) you will have more subnet out of that subnet (48). VLSM has to be done ONLY on UNUSED subnets or it wont work.
Example,
IP address = 172.16.32.0/20
Subnet = 255.255.240.0
by regular subnetting you get the following subnets:
# of subnets = 2^4-2=14
# of hosts = 2^12-2=4094
256-240 = 16
available subnets are : 16,32,48,64,80,up to subnet 224
Assuming you used the following subnets to address your network 16,32,48,64, NOW you have the rest of subnets (from 80 to 224) available for future use. Now you want to take advantge of VLSM to address your WAN connections so instead of using subnet 80 to do that (it will give you one subnet and 14 hosts) you will further subnet it (subnet 80) further
172.16.80.0/26
255.255.255.192
# of subnets = 2^2-2=2
# of hosts = 2^6-2=62
256-192=64
available subnets = 64,128,
available hosts in subnet 64 are hosts from 65 to 126, and on subnet 128 are from host 129 to host 190 total of 124 hosts.
Now you can see the difference, if we used our original subnet 80 we had ONE subnet with 14 hosts (from 81 to 94), after applying VLSM we had TWO subnets with 62 hosts/subnet for a total of 124.
I know subnet zero has to be considered in VLSM, how? I really don't know that, may be some one will give us some directions.
Please correct me if I am wrong.
HTH | |
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