Converting CIDR (Classless Inter-Domain Routing) to IP address range

CIDR notation represents an IP address range using a combination of an IP address and its associated network mask. Instead of specifying the network mask in IP format (xxx.xxx.xxx.xxx), CIDR notation uses number.

The format of CIDR notation is:

xxx.xxx.xxx.xxx/n

Where xxx.xxx.xxx.xxx is the address space, and n is the number of left most 1 bits in the network mask. 

For example, if 'n' is 18, then the corresponding network mask is: 255.255.3.0

There are online tool, which accepts input in CIDR format and outputs the IP range for example refer: http://www.ipaddressguide.com/cidr

Given below is a program I wrote in node.js, which produce the IP range for a given CIDR input:

Note: I had to design and implement network commands to manage Windows Azure Network using Windows Azure Tool for Node.js, as a part of implementation i used below logic to convert CIDR to IP range.

var addressSpace = // The first part of cidr value xxx-xxx-xxx-xxx

var cidr = // the second part of the cidr value n

var addressSpaceRange = {start:null, end:null};

var addressSpaceAsOctects = {

  'octect0': null,

  'octect1': null,

  'octect2': null,

  'octect3': null,

};

var networkMaskAsoctects = {

  'octect0': 0xff,

  'octect1': 0xff,

  'octect2': 0xff,

  'octect3': 0xff,

};

var ipv4Pattern = new RegExp(/^([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])$/);

var patternMatch = addressSpace.match(this.ipv4Pattern);

for (var i = 0; i < 4; i++) {

  addressSpaceAsOctects ['octect' + i] = parseInt(patternMatch[i + 1]);

}

var i = Math.floor(cidr / 8);

networkMaskAsoctects['octect' + i] = (0xff << (8 - cidr % 8)) & 0xff;

for (var j = i + 1; j < 4; j++) {

  networkMaskAsoctects['octect' + j] = 0;

}

addressSpaceRange.start =  {

  'octect0': addressSpaceAsOctects.octect0 & networkMaskAsoctects.octect0,

  'octect1': addressSpaceAsOctects.octect1 & networkMaskAsoctects.octect1,

  'octect2': addressSpaceAsOctects.octect2 & networkMaskAsoctects.octect2,

  'octect3': addressSpaceAsOctects.octect3 & networkMaskAsoctects.octect3,

};

addressSpaceRange.end = {

  'octect0': addressSpaceAsOctects.octect0 | (~(networkMaskAsoctects.octect0) & 0xff),

  'octect1': addressSpaceAsOctects.octect1 | (~(networkMaskAsoctects.octect1) & 0xff),

  'octect2': addressSpaceAsOctects.octect2 | (~(networkMaskAsoctects.octect2) & 0xff),

  'octect3': addressSpaceAsOctects.octect3 | (~(networkMaskAsoctects.octect3) & 0xff),
}