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

I ended up repurposing some PHP code I had found and tweaking to my needs. Below is the class I ended up with.

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RangeToCidr {
    private static final String IP_ADDRESS = "(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})\\.(\\d{1,3})";
    private static final Pattern addressPattern = Pattern.compile(IP_ADDRESS);

    public static List<String> rangeToCidrList(String istart, String iend)  {       
        int start = toInteger(istart);
        int end = toInteger(iend);

        List<String> result = new ArrayList<String>();

        while (end >= start) {
            int maxsize = imaxblock( start, 32);
            double x = (Math.log(end - start + 1) / Math.log(2) ) ;
            int maxdiff = (int) (Math.floor(32 - Math.floor(x)));

            String ip = intToIP(start);
            if (maxsize < maxdiff) {
                maxsize = maxdiff;
            }
            result.add( ip + "/" + (int)maxsize );
            start += Math.pow(2, (32-maxsize));
        }
        return result;
    }

    private static int toInteger(String address) {
        Matcher matcher = addressPattern.matcher(address);
        if (matcher.matches()) {
            return matchAddress(matcher);
        }
        else
            throw new IllegalArgumentException("Could not parse [" + address + "]");
    }

    private static int matchAddress(Matcher matcher) {
        int addr = 0;
        for (int i = 1; i <= 4; ++i) { 
            int n = (rangeCheck(Integer.parseInt(matcher.group(i)), -1, 255));
            addr |= ((n & 0xff) << 8*(4-i));
        }
        return addr;
    }

    private static int rangeCheck(int value, int begin, int end) {
        if (value > begin && value <= end) // (begin,end]
            return value;

        throw new IllegalArgumentException("Value [" + value + "] not in range ("+begin+","+end+"]");
    }

    private static String intToIP(int val) {
        int octets[] = new int[4];
        for (int j = 3; j >= 0; --j)
            octets[j] |= ((val >>> 8*(3-j)) & (0xff));

        StringBuilder str = new StringBuilder();
        for (int i =0; i < octets.length; ++i){
            str.append(octets[i]);
            if (i != octets.length - 1) {
                str.append("."); 
            }
        }
        return str.toString();
    }

    private static long imask(int t)    {
        return (long)(Math.pow(2, 32) - Math.pow(2, 32-t) ) ;
    }

    private static int imaxblock(long ibase, int tbit)  {
        while (tbit > 0)    {
            long im = imask(tbit-1);
            long imand = ibase & im ;
            if (imand != ibase) {
                break;
            }
            tbit--;
        }
        return tbit;
    }
}

I've got a few helper methods in there that, for the purposes of this question, clutter things up. But you can get the general idea.

Answer 2

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();     
    } 
}

Answer 3

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.

Answer 4

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]

Answer 5

The following CIDR blocks contain (not limited to) the range of addresses 1.1.1.111 - 1.1.1.120

/1 - /27

address   prefix   network    DirectedBroadcast 
1.1.1.111   /27    1.1.1.96   1.1.1.127
1.1.1.111   /26    1.1.1.64   1.1.1.127
1.1.1.111   /25    1.1.1.0    1.1.1.127
1.1.1.111   /24    1.1.1.0    1.1.1.255

etc.