Calculating The Number Of Subnets And Nodes - D-Link DES-3250TG User Manual

DES-3250TG Layer 2 Fast Ethernet Switch User's Guide
network (the broadcast address). This also applies to subnets.
A subnet address cannot be all "0"s or all "1"s. A 1-bit subnet
mask is also not allowed.

Calculating the Number of Subnets and Nodes

To calculate the number of subnets and nodes, use the formula
(2 – 2) where n = the number of bits in either the subnet mask
n
or the node portion of the IP address. Multiplying the number
of subnets by the number of nodes available per subnet gives
the total number of nodes for the entire network.
Example
00001010.00101010.01001001.11010010
address
11111111.11100000.00000000.00000000
00001010.00100000.00000000.00000000
00001010.00101010.11111111.11111111
Address
This example uses an 11-bit subnet mask. (There are three
additional bits added to the default Class A subnet mask). So
the number of subnets is:
3
2 – 2 = 8 – 2 = 6
Subnets of all "0"s and all "1"s are not allowed, so two subnets
are subtracted from the total.
The number of bits used in the node part of the address is 24 –
3 = 21 bits, so the total number of nodes is:
21
2 – 2 = 2,097,152 – 2 = 2,097,150
Multiplying the number of subnets times the number of nodes
gives 12,582,900 possible nodes.
Note that this is less than the 16,777,214 possible nodes that
an unsubnetted class A network would have.
10.42.73.210 Class A IP
255.224.0.0 Subnet Mask
10.32.0.0 Network Address
10.32.255.255 Broadcast
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