IPv4 Address Conservation
Because of the depletion of public IPv4 address space, making the most out of the available host addresses is a primary concern when subnetting IPv4 networks.
Note: The larger IPv6 address allows for much easier address planning and allocation than IPv4 allows. Conserving IPv6 addresses is not an issue. This is one of the driving forces for transitioning to IPv6.
Using traditional subnetting, the same number of addresses is allocated for each subnet. If all the subnets have the same requirements for the number of hosts, or if conserving IPv4 address space is not an issue, these fixed-size address blocks would be efficient. Typically, with public IPv4 addresses, that is not the case.
For example, the topology shown in the figure requires seven subnets, one for each of the four LANs, and one for each of the three connections between the routers.
Using traditional subnetting with the given address of 192.168.20.0/24, three bits can be borrowed from the host portion in the last octet to meet the subnet requirement of seven subnets. As shown in the figure, borrowing 3 bits creates 8 subnets and leaves 5 host bits with 30 usable hosts per subnet. This scheme creates the needed subnets and meets the host requirement of the largest LAN.
Basic Subnet Scheme
These seven subnets could be assigned to the LAN and WAN networks, as shown in the figure.
Although this traditional subnetting meets the needs of the largest LAN and divides the address space into an adequate number of subnets, it results in significant waste of unused addresses.
For example, only two addresses are needed in each subnet for the three WAN links. Because each subnet has 30 usable addresses, there are 28 unused addresses in each of these subnets. As shown in the figure, this results in 84 unused addresses (28×3).
Unused Addresses on WAN Subnets
Further, this limits future growth by reducing the total number of subnets available. This inefficient use of addresses is characteristic of traditional subnetting. Applying a traditional subnetting scheme to this scenario is not very efficient and is wasteful.
The variable-length subnet mask (VLSM) was developed to avoid wasting addresses by enabling us to subnet a subnet.
In all of the previous subnetting examples, the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses. As illustrated in the left side of the figure, traditional subnetting creates subnets of equal size. Each subnet in a traditional scheme uses the same subnet mask. As shown in the right side of the figure, VLSM allows a network space to be divided into unequal parts. With VLSM, the subnet mask will vary depending on how many bits have been borrowed for a particular subnet, thus the “variable” part of the VLSM.
VLSM is just subnetting a subnet. The same topology used previously is shown in the figure. Again, we will use the 192.168.20.0/24 network and subnet it for seven subnets, one for each of the four LANs, and one for each of the three connections between the routers.
The figure shows how network 192.168.20.0/24 subnetted into eight equal-sized subnets with 30 usable host addresses per subnet. Four subnets are used for the LANs and three subnets could be used for the connections between the routers.
Basic Subnetting Scheme