Explain how VLSM (Variable Length Subnet Masking) works.

Variable Length Subnet Masking (VLSM) is a technique used in IP address allocation to optimize the use of IP address space in a network. In traditional subnetting, all subnets within a network use the same subnet mask, leading to inefficient use of IP addresses. VLSM allows for the use of different subnet masks for different subnets within the same network, resulting in a more efficient allocation of IP addresses.

Here's how VLSM works:

  1. Understand Subnetting:
    Before diving into VLSM, it's crucial to have a good understanding of subnetting. Subnetting involves dividing an IP network into smaller, more manageable subnetworks. Each subnet has its own unique subnet address and range of host addresses.
  2. Define Subnetting Requirements:
    Identify the various subnets required in your network and determine the number of hosts each subnet needs to support. This information will help in selecting appropriate subnet mask lengths for each subnet.
  3. Allocate IP Addresses:
    With VLSM, you can use different subnet masks for different subnets based on their size and requirements. Larger subnets may have a shorter subnet mask to accommodate more hosts, while smaller subnets may have a longer subnet mask to conserve IP addresses.
  4. Example:
    Let's consider an example with the IP address This network has a default subnet mask of, allowing for 256 host addresses. Now, with VLSM, you might decide to create two subnets:By applying VLSM, you efficiently allocate IP addresses based on the actual needs of each subnet.
    • Subnet 1: Requires 100 hosts, so you might use a subnet mask of (/25). This provides 128 host addresses in this subnet.
    • Subnet 2: Requires 30 hosts, so you might use a subnet mask of (/27). This provides 32 host addresses in this subnet.
  5. Routing:
    Routers in the network need to support VLSM. Each router interface connecting to different subnets should be configured with the appropriate subnet mask. This ensures that the router can correctly route traffic between subnets with different subnet masks.

VLSM is especially useful in larger networks where different subnets may have significantly different numbers of hosts. It allows for more efficient use of IP address space and helps prevent the exhaustion of IP addresses in a network.