Publication date: Mar. 2016
33600 Rev B
15
Not all IP address is available in the sub-netted network. Two special
addresses are reserved. They are the addresses with all zero’s and all one’s host
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
10000000.00000001.00000010.1 0000000
128.1.2.128/25
25 bits
1 0000000
1 1111111
All 0s = 128.1.2.128
All 1s= 128.1.2.255
Subnet
Network
