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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 2-3
There will be a maximum of 254 effective nodes exist along the sub-netted
network. This network is considered to be a physical autonomous network that it
owns a network IP address which may look like 168.1.2.0.
With the subnet mask, a big network can be divided into smaller 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.