2.4 Variable-Length Subnet Masks
2.4.3 Calculating VLSM
As discussed previously, VLSMs allow you to subnet an already subnetted address. Consider, for example, that you need to assign subnetted address 172.6.32.0/20 ) to a network that has 10 hosts (allowing 12 bits for the host portion).

With this address, however, you get over 4,000 (212-2=4094) host addresses, so you would be wasting about 4,000 IP addresses. With VLSMs, you can further subnet address 172.6.32.0/20 to give you more network addresses and fewer hosts per network, which would provide you with more network addresses to grow your network. If, for example, you subnet 172.6.32.0/20 to 172.6.32.0/26, you can now have up to 1,022 (210-2) subnetworks, each of which could support about 62 (26-2) hosts.

Note VLSMs provide the capability to include more than one subnet mask within a network and the capability to subnet an already subnetted network address.

To further subnet 172.6.32.0/20 to 172.6.32.0/26 and gain five more network addresses, perform the following steps (see Figure ):

  1. Write 172.6.32.0 in binary form.
  2. Draw a vertical line between the 20th and 21st bits. 
  3. Draw a vertical line between the 26th and 27th bits.
  4. Calculate the five network addresses from lowest to highest in value. If necessary, refer to the IP address calculation tables in Figures and .

Note There are also VLSM calculators on the Web. The following URL is for the one offered by Cisco: http://www.cisco.com/techtools/ip_addr.html.

In Figure , the # Bits column indicates how many bits have been taken from the host address bits from the classful address. For example, the first line indicates that two bits have been masked off from the host address portion to be used as subnet bits. This creates a subnet value of 11000000 in the third byte (which has the decimal equivalent of 192).

The Mask column indicates the mask value in decimal once the subnet bits have been masked off. For example, 255.255.192.0 is the mask for a Class B address that is subnetted using two bits for the subnet portion. The Effective Subnets and Effective Hosts columns indicate how many different network and host numbers can be created with the bits masked off for the network portion and remaining host portion. Network and host numbers using all 1s or all 0s are not counted in these numbers.

Figure defines the subnet options using a Class C address that typically provides three bytes for the network portion and one byte for the host portion.