Archive for August, 2009


Lightweight IPv4


Published in: Development, Language, PHPPosted by Jdub7 on 08/03/09


Introduction

For a midsize project I wanted to store IP ranges in the database with the option to to also store CIDR blocks. CIDR blocks, though powerful are somewhat difficult for a typical user. Additional using them is not as precise for all ip ranges. There are a plethora of tools available on the internet that will do what I would like. However, incorporating these tools would not be practical. What I want is to mimic the functionality of these tools so that it can be easily imported into any of my projects. The most coveted tool for me would be to convert an IP range to a precise range of CIDR blocks. This required specific functionality not naturally provided in PHP.

  • Check for Valid Netmask
  • Check whether an IP address is within a CIDR block.
  • Take user input and a Netmask and make it into a valid CIDR block.
  • CIDR number into Netmask
  • Netmask to CIDR
  • Take an IP range and fit it into an exact range of CIDR blocks.

This presents some difficulty in that PHP’s network functions are not thorough enough. The revelation came when I realized that an IP address is merely a number. In fact the whole protocol is rooted in binary using very specific patterns. With that in mind I thought we could develop very light weight methods to solve our problem.

The Code

It is important to note that the methods provided are meant for IPv4 addresses only are only tested on a 32bit system. Also, I did not care to do much in the way of error checking, but doing so, like testing whether the CIDR number is unsigned and less than or equal to 32, should be trivial.

Though the solution I sought after would require PHP I didn’t limit myself to that language only. In fact the PHP code I found seemed inefficient. Most involved a number conversions or parsing the address using sprintf using loops and nested if statements. Indeed the most efficient code, which shouldn’t surprise most was in ANSI C. Bit Twiddling Hacks resource proved very useful.

About Binary

I am not attempting to teach binary math. Since the code does not “read like prose” a small amount of knowledge is required in order to understand the code. Some excellent resources are Wikipedia’s article on CIDR and PHP binary operators.