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

The open-source IPAddress Java library can do this for you. Disclaimer: I am the project manager of the IPAddress library.

Here is a sample method to do it:

static void toPrefixBlocks(String str1, String str2) {
    IPAddressString string1 = new IPAddressString(str1);
    IPAddressString string2 = new IPAddressString(str2);
    IPAddress one = string1.getAddress(), two = string2.getAddress();
    IPAddressSeqRange range = one.toSequentialRange(two);
    System.out.println("starting with range " + range);
    IPAddress blocks[] = range.spanWithPrefixBlocks();
    System.out.println("prefix blocks are " + Arrays.asList(blocks));
}

Using your example:

toPrefixBlocks("1.1.1.111","1.1.1.120");

The output is:

starting with range 1.1.1.111 -> 1.1.1.120
prefix blocks are [1.1.1.111/32, 1.1.1.112/29, 1.1.1.120/32]