Given a starting InetAddress and an ending InetAddress compute a raw subnet mask that will be used as the mask for the isOnNetwork() method. - Java Network

Java examples for Network:Network Address

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Description

Given a starting InetAddress and an ending InetAddress compute a raw subnet mask that will be used as the mask for the isOnNetwork() method.

Demo Code


//package com.java2s;
import java.net.InetAddress;

public class Main {
    /** Masks used to get bit values in a byte. */
    private static final byte[] BYTE_MASK = { 0x7f, 0x3f, 0x1f, 0x0f, 0x07,
            0x03, 0x01, 0x00 };//from  www. j a v  a  2  s  . c  om

    /**
     * Given a starting InetAddress and an ending InetAddress compute a 
     * raw subnet mask that will be used as the mask for the isOnNetwork()
     * method.
     * 
     * We use the following heuristic approach to calculating the subnet mask:
     * If the bits of the addresses are the same (start[0] = 0 and end[0] = 0; 
     * likewise start[0] = 1 and end[0] = 1), then the corresponding bit
     * of the subnet mask is 1; otherwise the corresponding bit of the subnet
     * mask is 0.  We evaluate each bit of the start and end addresses until
     * we find one that is different.  After this point, the rest of the 
     * subnet mask is 0 (represents the host part of the subnet mask).
     *
     * @param start Starting address.
     * @param end Ending address.
     * @return Raw subnet address.
     */
    public static byte[] getSubnetMask(InetAddress start, InetAddress end) {

        byte[] startAddr = start.getAddress();
        byte[] endAddr = end.getAddress();
        byte[] maskAddr = new byte[startAddr.length];

        for (int i = 0; i < maskAddr.length; i++) {

            for (int j = 0; j < BYTE_MASK.length; j++) {

                /*
                 * Flip the bits of the mask (so instead of 0111, we have 1000)
                 * to make the logic easier to follow.
                 */
                byte byteMask = (byte) ~BYTE_MASK[j];

                byte startAddrMask = (byte) (startAddr[i] & byteMask);
                byte endAddrMask = (byte) (endAddr[i] & byteMask);
                if (startAddrMask != endAddrMask) {
                    // End of network part of subnet, rest should already be zeros.
                    return maskAddr;
                }
                maskAddr[i] = (byte) (maskAddr[i] | byteMask);
            }
        }
        return maskAddr;
    }
}

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