Subnet Efficiency

When bits from the host field are "borrowed" to create subnets,some addresses are not available for hosts:
  1. Subnet 0 (subnet bits all 0) is used for a network address.
  2. Subnet 2n-1 (subnet bits all 1) is used for network broadcast.
On each subnet, some addresses are not available for hosts:
  1. Address 0 (host bits all 0) is used as a subnetwork address.
  2. Address 1 is used for a subnet gateway router.
  3. Address 2m-1 (host bits all 1) is used a subnetwork broadcast.
The number of hosts that a particular network can support is based ontotal bits available (Class A, 24 bits; B, 16; C, 8) andnumber of bits borrowed to form any subnets.

Borrowing 1 bit is impossible: both subnets (0 and 1) cannot supporthosts as shown above. Borrowing n-1 bits (where n is the total bitsavailable) is impossible:both hosts (0 and 1) are used as network and broadcast addresses.

The following Excel worksheet shows the total number of hosts eachclass of network can support based on all the possible subnettingschemes.


  1. If some addresses are inevitably lost, why use subnetting?
  2. Is there ever a good reason to use the schemes that are veryinefficient?
  3. Given enough addresses so there are enough addresses fornetwork expansion, is it better to provide for more subnetsor more hosts per subnet?

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© Copyright 1999 Guy T. Schafer
Last updated June, 1999