19
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 results.
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
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 existing in this sub-netted network
and is considered a physical network in an autonomous network. So it owns an IP address which may
look like 168.1.2.0.
With the subnet mask, a bigger network can be cut into smaller pieces. If we want to have more
than two independent networks, a partition to the network must be performed. In this case, subnet mask
must be applied.
