In Java, given an IP Address range, return the minimum list of CIDR blocks that covers the range

Question

I am having trouble with some of the logic in converting an IP Address range into a list of CIDR blocks. I do believe that this website is doing it right: http://ip2cidr.com

Answer 1

You need to understand binary numbers, nothing more.

An CIDR block is nothing else than a series of binary numbers with common prefix and all possible suffixes. Assume for the example below we had 8-bit IP-addresses, with classes /1, ... to /8.

In your case (ignoring the 1.1.1 for now), we write your numbers as binary numbers:

 1101111   - 111
 1110000   - 112
 1110001   - 113
   ...
 1110110   - 118
 1110111   - 119
 1111000   - 120

You'll see that all numbers have a common 11 prefix, but our list does not contain all these numbers. So we have to split it in two lists - one with 110 and one with 111. The first contains only one number, so we make a /8 block out of it (111/8).

The other list (from 112 to 120) contains not all numbers with 111 (since then it would go up to 127), so we split again - one list with 1110, the other with 1111. The first one is now the complete block 1110???? (or 112/4), the second one is only one single address, namely 11111000 (or 120/8).

So, now only extend to 32 bit instead of 8, and implement in Java, and you are ready.

In mathematical terms, one block always goes from x * 2^n to (x+1) * 2^n - 1, and we then use 32 - n as the block-size suffix. So you only need to find the next multiple of some power of two.