Question

How to split a network block to sub blocks in Java. I want to find out from a given 67.10/16 block, what blocks of /18 can I generate. For e.g. output should be this..

67.10.0/18
67.10.64/18
67.10.128/18
67.10.192/18

Or If I input 67.10/16 and the target cidr of /17 then the output should be

67.10.0/17
67.10.128/17

What is the best and efficient way of doing this.
Thank You

Answer 1

For example – subnet 67.10.1.1 /16 into four blocks.
In general:

Convert the base IP address to a number.

01000011 00001010 00000001 00000001 (1,124,729,089)

Convert the base mask to a number.

11111111 11111111 00000000 00000000 (4,294,901,760)

Mask the base IP with the mask giving the true base IP.

01000011 00001010 00000000 00000000 (1,124,728,832)

Convert the new mask (/18) to a number

11111111 11111111 11000000 00000000 (4,294,950,912)

XOR the original mask and the new mask to get the addition factor

00000000 00000000 11000000 00000000 (49,152)

Starting with the true base IP

for x = 0 to (2^(new cidr-old cidr))-1
 use **true base IP**
 true base IP = true base IP + addition factor
next x