Given a CIDR address, e.g. 192.168.10.0/24
- How to determine mask length? (
24
) - How to determine mask address? (
255.255.255.0
) - How to determine network address? (
192.168.10.0
)
Given a CIDR address, e.g. 192.168.10.0/24
24
)255.255.255.0
)192.168.10.0
)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
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());