VLSM ( Variable Length Subnet Mask )

VLSM ( Variable Length Subnet Mask ) is to make efficient use of classless inter-domain routing ( CIDR ) and route to control the routing table size, a network administrator uses advanced IP addressing technology, VLSM is one of the commonly used methods, can on the net hierarchical addressing, so that the most effective use of existing address space.

Brief introduction

VLSM ( Variable Length Subnet Mask ), RFC 1878 definition of a variable length subnet mask, VLSM to be in one of subnetting network in different parts of the use of different subnet mask. The internal network in different segments need different size in the case of very effective network.

The definition of VLSM: in order to effectively use a classless inter domain routing ( CIDR ) and route to control the routing table size, a network administrator uses advanced IP addressing technology, VLSM is one of the commonly used methods.

VLSM can couplet net hierarchical addressing, the advanced IP addressing technology allows the network administrator to the subnet division, so that the most effective use of existing address space.

How to use the VLSM

VLSM is relative to the class IP address. The first section is a network number ( eight ), B address before the two paragraph is the network number ( sixteen ), C in the first three paragraphs is the network number ( top twenty-four ). While the VLSM role is in the class IP address basis, from their host number part corresponding digit lending to do, is to increase the network number of digits. Various types of network can be used to divide the number of subnet: a twenty-four can borrow, B sixteen can borrow, C eight can borrow (can partition number is the host number number. In fact can not lend out, because the IP address must be on the part of the host, and the host number portion remaining one is meaningless, so in practice can borrow bits are in I wrote those figures minus 2, borrow bits for the network part).

This is a different size of network distribution mechanism, refers to a network can be configured in different mask. Development of variable length subnet mask idea is in each subnet retain sufficient host number at the same time, put a subnet is further divided into a plurality of small network for greater flexibility. If no VLSM, a subnet mask can only provide a network. This limits the number of hosts on the subnet number. In addition, VLSM is based on the bit, and the network is based on the 8 set of bits.

In the actual engineering practice, can further divide into three stages or more stages subnet. At the same time, to consider the use of all 0 and all 1 subnet to save network address space. A local area network using a 27 mask, then each subnet can support 30 host ( 2 ^ 5-2 = 30 ); while the WAN connections, each connection requires only 2 address, the ideal solution is to use 30 bit mask ( 2 ^ 2-2 = 2 ), but with the main categories of networks with the same mask constraint, WAN also must use 27 bit mask, which wastes a 28 address.