| A netmask is a 32-bit mask
used to divide an IP address into subnets and specify the networks
available hosts. In a netmask, two bits are
always automatically assigned. For example, in 255.255.225.0,
"0" is the assigned network address; and in
255.255.255.255, "255" is the assigned broadcast
address. The 0 and 255 are always assigned and cannot be used.
Below is an example of a netmask and an example of its binary
conversion.
| Netmask: |
255. |
255. |
255. |
255 |
| Binary Conversion: |
11111111 |
11111111 |
11111111 |
11111111 |
| Netmask length |
8 |
16 |
24 |
32 |
Counting out the bits in the binary conversion allows you to
determine the netmask length. In the above example you have an example
of a 32-bit address. However, this address is a broadcast
address and does not allow any hosts (computers or other network devices)
to be connected to it.
A commonly used netmask is a 24-bit netmask as seen below.
| Netmask: |
255. |
255. |
255. |
0 |
| Binary Conversion: |
11111111 |
11111111 |
11111111 |
00000000 |
| Netmask length |
8 |
16 |
24 |
-- |
Using a 24-bit netmask the network would be capable of 2,097,150
networks or 254 different hosts with an IP range of 192.0.1.x -
223.255.254.x. This is commonly plenty of addresses for one network.
To determine the capable amount of networks a netmask is able to
support is a simple formula. With the understanding that the netmask
length is 24, subtract three from that number, therefore 24-3 =
21. Once this number is determined, take 2 to the x power - 2
(where x is the number you just determined). Which would be 2^21
- 2 = 2,097,150. You are subtracting two from this number because of
the broadcast and network addresses that are already being used.
To determine the amount of hosts a netmask is capable of supporting
is similar to the above. As you see by the above chart, we have eight
zeros. This number is similar to the 21 we determined earlier.
Therefore, 2 to the x power - 2. (where x is the number of zeros in the
netmask). This would be 2^8 - 2 = 254. Again, two is
subtracted from this number to account for the broadcast and network
addresses.
Below is a breakdown of each of the commonly used network classes:
| Type of Class |
Netmask length |
# of networks |
# of hosts |
Netmask |
| Class A |
8 |
126 |
16,777,214 |
255.0.0.0 |
| Class B |
16 |
16,382 |
65,534 |
255.255.0.0 |
| Class C |
24 |
2,097,150 |
254 |
255.255.255.0 |
Also see: Binary, IP
address, Network definitions, Subnet
Mask
|
|
| Resolved |
Were you able to locate the answer to your questions? |
|
|
