How to determine the number of IP addresses available for a given subnet range?
Quite common query most of us has and will try to explain a bit on how this can be evaluated.
Each IP address is a represent as 32 bits, each set of 8 bits separated by a dot (.)
10.0.0.1 stands for
0000 1010.0000 0000.0000 0000.0000 0001
Bits are computed by adding values of positions
POSITIONS 8 7 6 5 4 3 2 1
Value 2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0 i.e.
Computed Value 128 64 32 16 8 4 2 1
10 can be formed only when positions 4 and 2 have a value i.e. 0000 1010
Few Examples
0 = 0000 0000 (0)
1 = 0000 0001 (1)
2 = 0000 0010 (2)
3 = 0000 0011 (2 + 1)
40 = 0010 1000 (32 + 8)
255 = 1111 1111 (128+64+32+16+8+4+2+1)
Subnet Range
Eg: 10.0.0.0/16 (16 is the CIDR Range - Classless Inter-domain Routing)
This subnet range indicates that, of all the 32 bits of an IP, the last 16 (32-16) bits can be used to define the Host Names, while the first 16 bits are Network Identifiers.
The range of ips for this subnet would be
10.0.(0-255).(0-255) which would be about 2^16 i.e. 65536 IPs.
Similarly, 10.0.0.0/24 means, the last 8 (32 - 24) bits can be changed to form host names i.e.
10.0.0.(0-255), which would be about 2^8 Ips i.e. 256 IPs
10.0.0.0/28 means, the last 4 (32 - 28) bits can be changed to form host names i.e.
10.0.0.(0-15), which would be about 2^4 Ips i.e. 16 IPs
An alternate way to calculate the IP count --
For a subnet 10.0.1.0/24
Total bits = 32
Usable = 32 - 24 = 8 (24 is the CIDR range)
Total IPs = 2^8 (8 is the usable bit count) = 256
Example 2: For subnet 10.0.1.0/30
Total bits = 32
Usable = 32 - 30 = 2 (30 is the CIDR range)
Total IPs = 2^2 (2 is the usable bit count) = 4
i.e. 10.0.1.0, 10.0.1.1, 10.0.1.2, 10.0.1.3
Hope this helps !!!
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