How to check if an IP address is from a particular network/netmask in Java?

Question

I need to determine if given IP address is from some special network in order to authenticate automatically.

Answer 1

here is an Version that works with IPv4 and IPv6 one with Prefix and one with Network Mask.

/**
 * Check if IP is within an Subnet defined by Network Address and Network Mask
 * @param  ip
 * @param  net
 * @param  mask
 * @return
 */
public static final boolean isIpInSubnet(final String ip, final String net, final int prefix) {
    try {
        final byte[] ipBin   = java.net.InetAddress.getByName(ip  ).getAddress();
        final byte[] netBin  = java.net.InetAddress.getByName(net ).getAddress();
        if(ipBin.length  != netBin.length ) return false;
        int p = prefix;
        int i = 0;
        while(p>=8) { if(ipBin[i] != netBin[i] ) return false; ++i; p-=8; }
        final int m = (65280 >> p) & 255;
        if((ipBin[i] & m) != (netBin[i]&m) ) return false;

        return true;
    } catch(final Throwable t) {
        return false;
    }
}

/**
 * Check if IP is within an Subnet defined by Network Address and Network Mask
 * @param  ip
 * @param  net
 * @param  mask
 * @return
 */
public static final boolean isIpInSubnet(final String ip, final String net, final String mask) {
    try {
        final byte[] ipBin   = java.net.InetAddress.getByName(ip  ).getAddress();
        final byte[] netBin  = java.net.InetAddress.getByName(net ).getAddress();
        final byte[] maskBin = java.net.InetAddress.getByName(mask).getAddress();
        if(ipBin.length  != netBin.length ) return false;
        if(netBin.length != maskBin.length) return false;
        for(int i = 0; i < ipBin.length; ++i) if((ipBin[i] & maskBin[i]) != (netBin[i] & maskBin[i])) return false;
        return true;
    } catch(final Throwable t) {
        return false;
    }
}