VLSM (Variable Length Subnet Mask)

Internet Service providers could face a scenario wherever they have to apportion ip subnets of various sizes as per the need of client. One client could raise class C subnet of three ip addresses and another could elicit ten IPs. For an ISP, it’s not possible to divide the ip addresses into fixed size subnets, rather he might want to subnet the subnets in such the simplest way ip in minimum wastage of scientific discipline addresses. To solve this problem, we may use VLSM.

For example, associate administrator have 192.168.1.0/24 network. The suffix /24 (pronounced as “slash 24″) tells the quantity of bits used for network address. during this example, the administrator has 3 different|completely different} departments with different number of hosts. sales force has a hundred computers, Purchase department has fifty computers, Accounts has twenty five computers and Management has five computers. In CIDR, the subnets square measure of fixed size. exploitation constant methodology the administrator cannot fulfill all the necessities of the network.

The following procedure shows however VLSM may be employed in order to apportion department-wise ip addresses as mentioned within the example.

VLSM Step – 1

Make an list of Subnets potential.

Subnet Mask Slash Notation Hosts/Subnet
255.255.255.0 /24 254
255.255.255.128 /25 126
255.255.255.192 /26 62
255.255.255.224 /27 30
255.255.255.240 /28 14
255.255.255.248 /29 6
255.255.255.252 /30 2

VLSM Step – 2

Sort the necessities of IPs in descending order (Highest to Lowest).

Sales 100

Purchase 50

Accounts 25

Management 5

VLSM Step – 3

Allocate the highest range of IPs to the highest demand, therefore let’s assign 192.168.1.0 /25 (255.255.255.128) to the sales force. This ip subnet with Network number 192.168.1.0 has 126 valid Host ip addresses that satisfy the need of the sales force. The subnet mask used for this subnet has 10000000 because the last octet.

VLSM Step – 4

Allocate the next highest vary, therefore let’s assign 192.168.1.128 /26 (255.255.255.192) to the acquisition department. This ip subnet with Network range 192.168.1.128 has sixty two valid Host ip Addresses which might be simply appointed to all or any the PCs of the acquisition department. The subnet mask used has 11000000 within the last octet.

VLSM Step – 5

Allocate the next highest vary, i.e. Accounts. the need of twenty five IPs may be consummated with 192.168.1.192 /27 (255.255.255.224) ip subnet, that contains thirty valid host IPs. The network range of Accounts department are going to be 192.168.1.192. The last octet of subnet mask is 11100000.

VLSM Step – 6

Allocate the next highest vary to Management. The Management department contains solely five computers. The subnet 192.168.1.224 /29 with the Mask 255.255.255.248 has specifically 6 valid host ip addresses. therefore this may be appointed to Management. The last octet of the subnet mask can contain 11111000.

By exploitation VLSM, the administrator will subnet the ip subnet in such the simplest way that least range of ip addresses square measure wasted. Even once assignment IPs to each department, the administrator, during this example, remains left with lots of ip addresses that wasn’t potential if he has used CIDR.

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