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/

I would like to pass in a starting IP address and an ending IP address and have the java spit out the minimum list of CIDR blocks required to cover only the range passed in and nothing more.

For instance, if I pass in a start address of 1.1.1.111 and an end address of 1.1.1.120, I would expect to get in return: 1.1.1.111/32 1.1.1.112/29 1.1.1.120/32

(with the /32 indicating a single address.)

Answer 1

My last answer had some bugs in it that came about when the first octet of the IP address was too big. This one works better. Lifted almost entirely from here: http://facedroid.blogspot.com/2010/06/ip-range-to-cidr.html

import java.util.ArrayList;
import java.util.List;

public class RangeToCidr {
    public static List<String> range2cidrlist( String startIp, String endIp ) {         
        long start = ipToLong(startIp);         
        long end = ipToLong(endIp);           

        ArrayList<String> pairs = new ArrayList<String>();         
        while ( end >= start ) {             
            byte maxsize = 32;             
            while ( maxsize > 0) {                 
                long mask = CIDR2MASK[ maxsize -1 ];                 
                long maskedBase = start & mask;                 

                if ( maskedBase != start ) {                     
                    break;                 
                }                 

                maxsize--;             
            }               
            double x = Math.log( end - start + 1) / Math.log( 2 );             
            byte maxdiff = (byte)( 32 - Math.floor( x ) );             
            if ( maxsize < maxdiff) {                 
                maxsize = maxdiff;             
            }             
            String ip = longToIP(start);             
            pairs.add( ip + "/" + maxsize);             
            start += Math.pow( 2, (32 - maxsize) );         
        }         
        return pairs;     
    }       

    public static final int[] CIDR2MASK = new int[] { 0x00000000, 0x80000000,             
        0xC0000000, 0xE0000000, 0xF0000000, 0xF8000000, 0xFC000000,             
        0xFE000000, 0xFF000000, 0xFF800000, 0xFFC00000, 0xFFE00000,             
        0xFFF00000, 0xFFF80000, 0xFFFC0000, 0xFFFE0000, 0xFFFF0000,             
        0xFFFF8000, 0xFFFFC000, 0xFFFFE000, 0xFFFFF000, 0xFFFFF800,             
        0xFFFFFC00, 0xFFFFFE00, 0xFFFFFF00, 0xFFFFFF80, 0xFFFFFFC0,             
        0xFFFFFFE0, 0xFFFFFFF0, 0xFFFFFFF8, 0xFFFFFFFC, 0xFFFFFFFE,             
        0xFFFFFFFF };       

    private static long ipToLong(String strIP) {         
        long[] ip = new long[4];         
        String[] ipSec = strIP.split("\\.");         
        for (int k = 0; k < 4; k++) {             
            ip[k] = Long.valueOf(ipSec[k]);         
        }         

        return (ip[0] << 24) + (ip[1] << 16) + (ip[2] << 8) + ip[3];     
    }       

    private static String longToIP(long longIP) {         
        StringBuffer sb = new StringBuffer("");         
        sb.append(String.valueOf(longIP >>> 24));         
        sb.append(".");         
        sb.append(String.valueOf((longIP & 0x00FFFFFF) >>> 16));         
        sb.append(".");         
        sb.append(String.valueOf((longIP & 0x0000FFFF) >>> 8));         
        sb.append(".");         
        sb.append(String.valueOf(longIP & 0x000000FF));   

        return sb.toString();     
    } 
}