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? (255.255.255.0)
• How to determine network address? (192.168.10.0)

Answer 1

It is covered by apache utils.

See this URL: http://commons.apache.org/proper/commons-net/apidocs/org/apache/commons/net/util/SubnetUtils.html

String subnet = "192.168.0.3/31"; SubnetUtils utils = new SubnetUtils(subnet);  utils.getInfo().isInRange(address) 

Note: For use w/ /32 CIDR subnets, for exemple, one needs to add the following declaration :

utils.setInclusiveHostCount(true); 

Answer 2

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());