Subnet Masking; Calculating The Number Of Subnets And Nodes - D-Link DGS-3308 Series User Manual

8-port Gigabit Ethernet Switch User's Guide

Subnet Masking

A subnet mask can be applied to an IP address to identify the network and the node parts of the address. A bitwise logical
AND operation between the IP address and the subnet mask results in the Network Address.
For example:
00001010.00101010.01001001.11010010 10.42.73.210
11111111.00000000.00000000.00000000 255.0.0.0
00001010.00000000.00000000.00000000 10.0.0.0 Network Address
The Default subnet masks are:
Class A – 11111111.00000000.00000000.00000000
Class B – 11111111.11111111.00000000.00000000
Class C – 1111111.11111111.11111111.00000000
Additional bits can be added to the default subnet mask for a given Class to further subnet a network. When a bitwise
logical AND operation is performed between the subnet mask and the IP address, the result defines the Subnet Address.
Some restrictions apply to subnet addresses. Addresses of all "0"s and all "1"s are reserved for the local network (when a
host does not know its network address) and for all hosts on the 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
mask 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
11111111.11100000.00000000.00000000
00001010.00100000.00000000.00000000
00001010.00101010.11111111.11111111
This example uses an 11-bit subnet mask. (There are 3 additional bits added to the default Class A subnet mask). So the
number of subnets is:
2 – 2 = 8 – 2 = 6
3
Subnets of all "0"s and all "1"s are not allowed, so 2 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:
2 – 2 = 2,097,152 – 2 = 2,097,150
21
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.
Subnetting reduces the number of possible nodes for a given network, but increases the segmentation of the network.
Class A IP address
Class A Subnet Mask
– 2) where n = the number of bits in either the subnet
n
10.42.73.210 Class A IP address
255.224.0.0 Subnet Mask
10.32.0.0 Network Address
10.32.255.255 Broadcast Address
21
255.0.0.0
255.255.0.0
255.255.255.0