Question

I’m dealing with a problem to partition specific IP address ranges (in Java). Suppose I have:

startIP    endIP
1.2.3.4    1.2.5.6

and I need cut this range into every [0, 255] interval so we have:

startIP    endIP
1.2.3.4    1.2.3.255
1.2.4.0    1.2.4.255
1.2.5.0    1.2.5.6

I’m looking at this problem more or less like partitioning a decimal range for instance from 7 to 26 and we have [7, 9], [10, 19], [20, 26]. The only difference is we are dealing with 256-simal numbers. Any ideas, folks? Thank you!

Answer 1

Some sample code (assumes the initial translation to a int[] is done outside) and which doesn’t really return useful data (just prints). But the idea is there.

public class Convertor {
    public static void convert(int []a, int []b) {
        int left = (a[0] << 24) | (a[1] << 16) | (a[2] << 8) | a[3];
        int right = left | 0xff;
        int end = (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3];
        while ( right < end ) {
            System.out.printf("%s -> %s\n", toIp(left), toIp(right));
            left = right + 1; right += 256;
        }
        System.out.printf("%s -> %s\n", toIp(left), toIp(end));
    }

    private static String toIp(int value) {
        return String.format("%d.%d.%d.%d",
                             value >> 24 & 0xFF,
                             value >> 16 & 0xFF,
                             value >> 8 & 0xFF,
                             value & 0xFF);
    }
}

call site:

Convertor.convert(new int[]{1, 2, 3, 4}, new int[]{1, 2, 5, 6});

output:

1.2.3.4 -> 1.2.3.255
1.2.4.0 -> 1.2.4.255
1.2.5.0 -> 1.2.5.6