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In this diagram, you can see the subnet mask with 25-bit long,
255.255.255.128, contains 126 members in the sub-netted network. Another is that
the length of network prefix equals the number of the bit with 1s in that subnet mask.
With this, you can easily count the number of IP addresses matched. The following
table shows the result.
Prefix Length No. of IP matched No. of Addressable IP
/32 1 -
/31 2 -
/30 4 2
/29 8 6
/28 16 14
/27 32 30
/26 64 62
/25 128 126
/24 256 254
/23 512 510
/22 1024 1022
/21 2048 2046
/20 4096 4094
/19 8192 8190
/18 16384 16382
/17 32768 32766
/16 65536 65534
Table 2-4
According to the scheme above, a subnet mask 255.255.255.0 will partition a
network with the class C. It means there will have a maximum of 254 effective
nodes existed in this sub-netted network and is considered a physical network in an
autonomous network. So it owns a network IP address which may looks like
168.1.2.0.
With the subnet mask, a bigger network can be cut into small pieces of
network. If we want to have more than two independent networks in a worknet, a
partition to the network must be performed. In this case, subnet mask must be
applied.