Converting CIDR address to subnet mask and network address

Question

Given a CIDR address, e.g. 192.168.10.0/24

• How to determine mask length? (24)
• How to determine mask address? (255.255.

Answer 1

This is how you would do it in Java,

    String[] parts = addr.split("/");
    String ip = parts[0];
    int prefix;
    if (parts.length < 2) {
        prefix = 0;
    } else {
        prefix = Integer.parseInt(parts[1]);
    }
    int mask = 0xffffffff << (32 - prefix);
    System.out.println("Prefix=" + prefix);
    System.out.println("Address=" + ip);

    int value = mask;
    byte[] bytes = new byte[]{ 
            (byte)(value >>> 24), (byte)(value >> 16 & 0xff), (byte)(value >> 8 & 0xff), (byte)(value & 0xff) };

    InetAddress netAddr = InetAddress.getByAddress(bytes);
    System.out.println("Mask=" + netAddr.getHostAddress());

Answer 2

The algorithm is in pseudo code (actually PHP), you can translate it to java yourself.
Algoritm from here.

//$ipNetmask = "192.168.1.12/30";
list($ip, $netmask) = split( "/", $ipNetmask );
$ip_elements_decimal = split( "[.]", $ip );
$netmask_result="";
for($i=1; $i <= $netmask; $i++) {
  $netmask_result .= "1";
}
for($i=$netmask+1; $i <= 32; $i++) {
    $netmask_result .= "0";
}
$netmask_ip_binary_array = str_split( $netmask_result, 8 );
$netmask_ip_decimal_array = array();
foreach( $netmask_ip_binary_array as $k => $v ){
    $netmask_ip_decimal_array[$k] = bindec( $v ); // "100" => 4
    $network_address_array[$k] = ( $netmask_ip_decimal_array[$k] & $ip_elements_decimal[$k] );
}
$network_address = join( ".", $network_address_array );

// ------------------------------------------------
           // TCP/IP NETWORK INFORMATION
// ------------------------------------------------
// IP Entered = ..................: 192.168.1.12
// CIDR = ........................: /30
// Netmask = .....................: 255.255.255.252
// Network Address = .............: 192.168.1.12

// Broadcast Address = ...........: 192.168.1.15
// Usable IP Addresses = .........: 2
// First Usable IP Address = .....: 192.168.1.13
// Last Usable IP Address = ......: 192.168.1.14

Answer 3

The IPAddress Java library supports both IPv4 and IPv6 in a polymorphic manner including subnets. The javadoc is available at the link. Disclaimer: I am the project manager.

All the use cases you listed are supported for both IPv4 and Ipv6 transparently.

    String str = "192.168.10.0/24";
    IPAddressString addrString = new IPAddressString(str);
    try {
         IPAddress addr = addrString.toAddress();
         Integer prefix = addr.getNetworkPrefixLength(); //24
         IPAddress mask = addr.getNetwork().getNetworkMask(prefix, false);//255.255.255.0
         IPAddress networkAddr = addr.mask(mask);  //192.168.10.0
         IPAddress networkAddrOtherWay = addr.getLower().removePrefixLength(); //192.168.10.0

         ...
    } catch(AddressStringException e) {
        //e.getMessage provides validation issue
    }

Answer 4

You can use org.springframework.security.web.util.IpAddressMatcher from Spring Framework.

Answer 5

Following Yuriy's answer: To get the whole range of ip addresses, the Apache Java class SubnetUtils offers the following methods:

String[] addresses = utils.getInfo().getAllAddresses();

To download the jar containing the class go to: http://repo1.maven.org/maven2/commons-net/commons-net/3.0.1/commons-net-3.0.1.jar

The source code: http://svn.apache.org/viewvc/commons/proper/net/trunk/src/main/java/org/apache/commons/net/util/SubnetUtils.java?view=markup

Maven id:

<groupId>commons-net<groupId>
<artifactId>commons-net<artifactId>
<version>3.0.1<version>

Answer 6

Here is a simple Groovy example

def cidrAddress = '192.168.10.0/24'
def cidrAddressList = cidrAddress.tokenize("\\/")
def baseIPAddress = cidrAddressList.first()
def cidrIPMask = cidrAddressList.last().toInteger()
def netMaskList = []
Integer fullOctets = cidrIPMask.intdiv(8)
fullOctets.times {netMaskList.add('255')}
def remainder = cidrIPMask % 8
netMaskList.add((256 - (2 ** (8 - remainder))).toString())
netMaskList.addAll(['0','0','0','0'])
def netMask = netMaskList.flatten().getAt(0..3).join('.')
return [cidrAddress,baseIPAddress,cidrIPMask,netMask]