21
number. For example, an IP address 128.1.2.128, what IP address reserved will be
looked like? All 0s mean the network itself, and all 1s mean IP broadcast.
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
10000000.00000001.00000010.1 0000000
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