Additional bits can be added to the default subnet mask for a given Class to further subnet, or break down, a network. When a bitwise logical AND operation is performed between the subnet mask and IP address, the result defines the

To calculate the number of subnets or nodes, use the formula (2^{n}-2) where
n = number of bits in either field, and 2^{n} represents 2 raised to the nth power. Multiplying the number of subnets by the
number of nodes available per subnet gives you the total number of nodes available
for your class and subnet mask. Also, note that although subnet masks with non-contiguous
mask bits are allowed, they are not recommended.

Example:

10001100.10110011.11011100.11001000 140.179.220.200 IP Address 11111111.11111111.11100000.00000000 255.255.224.000 Subnet Mask -------------------------------------------------------- 10001100.10110011.11000000.00000000 140.179.192.000 Subnet Address 10001100.10110011.11011111.11111111 140.179.223.255 Broadcast Address

In this example a **3 bit subnet mask** was
used. There are 6 (2^{3}-2) subnets available with this size mask (remember that
subnets with all 0's and all 1's are not allowed). Each subnet has 8190 (2^{13}-2) nodes.
Each subnet can have nodes assigned to any address between the Subnet address
and the Broadcast address. This gives a total of 49,140 nodes for the entire
class B address subnetted this way. Notice that this is less than the 65,534
nodes an unsubnetted class B address would have.

You can calculate the Subnet Address by performing a bitwise logical AND operation between the IP address and the subnet mask, then setting all the host bits to **0**s. Similarly, you can calculate the *Broadcast Address* for a subnet by performing the same logical AND between the IP address and the subnet mask, then setting all the host bits to **1**s. That is how these numbers are derived in the example above.

Subnetting always reduces the number of possible nodes for a given network. There are complete subnet tables available here for Class A, Class B and Class C. These tables list all the possible subnet masks for each class, along with calculations of the number of networks, nodes and total hosts for each subnet.

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Updated January 31, 2007 .

Copyright © 1996-2007 by Ralph Becker < > send me Feedback!