Introduction to Subnetting and VLSM Topic for CSE Technical Seminar:

Every system has given a unique IP address consisting of 32 bits written in dotted decimal format. This 32 bit IP address is divided into two parts: first 0-15bits represents network number and from16-31bits represents host number. The IP address consists of binary numbers Following is the example of an IP address:

         40     .179       . 220          .200



An IP address consists of network number and host number therefore subnetting is dividing the network number part classes into small pieces. In subnetting bits are borrowed from host ID part to the network ID.The IP address is classified into 5 different classes. Those are Class A, Class B, Class C, Class D and Class E. The value ranges of these classes starting from class A are- 0 to 126 (127 is loop back), 128 to 191, 192 to 223, 224 to 239 and 240 to 255 respectively. To make the networks class A, Class B and Class C are used, for multicasting class D addresses are used and class E addresses are reserved for future use. IP address has two-level hierarchy in which network ID and host ID are present in subnetting the host ID is divided into subnet ID and host ID thus it forms three-level hierarchy.

The IP address for network and broadcasting is different, for network address all the bits present in host number are set to ‘0’ and for broadcast all the bits of host are set to ‘1’. Here we need not to consider the length of the host section. 

Custom subnet mask: 

Customized subnetting means dividing the point between subnet ID and host ID according to our choice. Custom subnet mask is a subnet mask that we use to create the customized subnet. 

Disadvantage of Subnetting: 

 In subnetting the host ID is partitioned into subnet ID and Host ID, the subnet ID represents IP addresses are interpreted and used for routing. Here all the addresses may not be used so that so many addresses may be wasted. Here is the solution to overcome this problem i.e.; VLSM. 

VLSM (Variable Length Subnet Mask): 

VLSM is allocating the address resources to the subnets based on the requirement simply VLSM is subnetting the subnets of variable size. Therefore VLSM is more flexible than the subnetting and reduces the wastage of IP address resources.

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