VLSM

VLSM (Variable Length Subnet Masking) helps improve the use of IP address space. Using a variable length subnet mask (VLSM), we can assign LAN and WAN segments without wasting IP Addresses. As shown in the scenario in the Figure below, the hosts in each subnetwork will be assigned a subnet according to the number of hosts to decrease the waste of IP addresses.

Each subnet’s first host IPv4 address is assigned to the router’s LAN interface. The routers’ WAN interfaces are assigned the IP addresses and mask for the /30 subnets. Hosts on each subnet will have a host IPv4 address from that subnet’s range of host addresses and an appropriate mask. Hosts will use the address of the attached router LAN interface as the default gateway address. The table below is the addressing scheme for the network mentioned above scenario.

In the previous lesson, we have assigned the one subnet to each network. Each network contains 126 usable hosts. You can see that many IPs will waste in-network human resources, quality control, and between the WAN ports of the routers. So, we can ignore the waste of IP addresses using VLSM subnetting.

First, we can assign the first three subnets of the previous lesson to the management, finance, and sales departments because the number of hosts in these LANs is between 64 and 128. Therefore, we need a network with /25 prefixes, which can accommodate 126 hosts.

We can subnet the Net-3 into /26, /27, and 8x /30 networks. The network with the/26 prefix can accommodate up to 62 hosts, and the network with the/27 prefix can accommodate up to 30 hosts. In comparison, the network with the/30 prefix is generally used for point-to-point connectivity. The figure below illustrates the assigned IP addresses to the scenario we discussed in the previous lesson. Still, you can see that we have saved many IP addresses for future use.

VLSM Chart

An addressing chart is very important for identifying the blocks of addresses already used and available for future use, as shown in the table below. The VLSM chart helps the network administrator to avoid assigning addresses that have previously been allocated.