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الموضوع : ماهو VLSM
Variable Length Subnet Mask
VLSM is a method of creating different-sized subnets in order to conserve IP
addresses. When you use fixed subnet mask subnetting on an internetwork that
has subnets with different requirements for the maximum number of hosts, a
large proportion of the addresses may be wasted. By using VLSM, you can
allocate the appropriate number of IP addresses to each subnet rather than use
fixed-length subnet masks.
Subnetting was originally used to subdivide a class-based network ID into a
series of equal-sized subnets. For example, a 4-bit subnetting of a class B
network ID produced 16 equal-sized subnets. In actuality, the number of hosts
per subnet is rarely if ever of equal size. This inequality results in many wasted
IP addresses.
Subnetting does not require equal-sized subnets, so you can conserve IP
addresses by using VLSM to create different-sized subnets that best match the
number of hosts in each subnet. VLSM is a recursive process that uses
subnetting to subnet network IDs that are already subnetted. The process
continues until you have unique subnet IDs that waste as few IP addresses per
subnet as possible. Because all subnetted network IDs are unique, they can be
distinguished from each other by their corresponding subnet mask.

How to use VLASM:You can reduce the number of IP addresses that remain unused by using
VLSM.
For example, given the class-based network ID of 157.54.0.0/16, a required
configuration is one subnet with up to 32,000 hosts, 15 subnets with up to 2,000
hosts, and eight subnets with up to 250 hosts.
Procedure for one
subnet with up to 32,000 hosts :
To achieve a requirement of one subnet with approximately 32,000 hosts,
subnet 1 bit of the class-based network ID of 157.54.0.0. This produces 2
subnets---157.54.0.0/17 and 157.54.128.0/17. This subnetting allows up to
32,766 hosts per subnet. 157.54.0.0/17 is chosen as the network ID, which
fulfills the requirement.

The following table shows one subnet with up to 32,766 hosts per subnet.
Subnet-Number Network ID (Dotted Decimal) Network ID (Network Prefix)
1 157.54.0.0, 255.255.128.0 157.54.0.0/17

To achieve a requirement of 15 subnets with approximately 2,000 hosts, subnet
4 bits of the subnetted network ID of 157.54.128.0/17. This produces 16
subnets (157.54.128.0/21, 157.54.136.0/21 through 157.54.240.0/21,
157.54.248.0/21), allowing up to 2,046 hosts per subnet. The first 15 subnetted
network IDs (157.54.128.0/21 through 157.54.240.0/21) are chosen as the
network IDs, which fulfills the requirement.
The following table shows 15 subnets with up to 2,046 hosts per subnet.
Subnet-Number Network ID (Dotted Decimal) Network ID (Network Prefix)
1 157.54.128.0, 255.255.248.0 157.54.128.0/21
2 157.54.136.0, 255.255.248.0 157.54.136.0/21
3 157.54.144.0, 255.255.248.0 157.54.144.0/21
4 157.54.152.0, 255.255.248.0 157.54.152.0/21
5 157.54.160.0, 255.255.248.0 157.54.160.0/21
6 157.54.168.0, 255.255.248.0 157.54.168.0/21
7 157.54.176.0, 255.255.248.0 157.54.176.0/21
8 157.54.184.0, 255.255.248.0 157.54.184.0/21
9 157.54.192.0, 255.255.248.0 157.54.192.0/21
10 157.54.200.0, 255.255.248.0 157.54.200.0/21
11 157.54.208.0, 255.255.248.0 157.54.208.0/21
12 157.54.216.0, 255.255.248.0 157.54.216.0/21
13 157.54.224.0, 255.255.248.0 157.54.224.0/21
14 157.54.232.0, 255.255.248.0 157.54.232.0/21
15 157.54.240.0, 255.255.248.0 157.54.240.0/21

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