VLSM allows the use of different masks for each subnet. After a network address is subnetted, those subnets can be further subnetted. VLSM is simply subnetting a subnet. VLSM can be thought of as sub-subnetting.
The figure shows the network 10.0.0.0/8 that has been subnetted using the subnet mask of /16, which makes 256 subnets. That is 10.0.0.0/16, 10.1.0.0/16, 10.2.0.0/16, …, 10.255.0.0/16. Four of these /16 subnets are displayed in the figure. Any of these /16 subnets can be subnetted further.
Click the Play button in the figure to view the animation. In the animation:
- The 10.1.0.0/16 subnet is subnetted again with the /24 mask.
- The 10.2.0.0/16 subnet is subnetted again with the /24 mask.
- The 10.3.0.0/16 subnet is subnetted again with the /28 mask
- The 10.4.0.0/16 subnet is subnetted again with the /20 mask.
Individual host addresses are assigned from the addresses of "sub-subnets". For example, the figure shows the 10.1.0.0/16 subnet divided into /24 subnets. The 10.1.4.10 address would now be a member of the more specific subnet 10.1.4.0/24.