June 07, 2011, 5:01 PM — The Internet has been rolling along for decades on the strength of IPv4 and its numbering system, which has supplied billions of addresses. As long as more addresses were available, few people thought about them. But the booming popularity of the Internet has finally soaked up nearly all those fresh numbers: In February, the Internet Corporation for Assigned Names and Numbers (ICANN) allocated the last of its unused large blocks of IPv4 addresses to regional Internet registries. On Wednesday, World IPv6 Day will turn the new protocol on at hundreds of companies, agencies and universities for testing. Suddenly, IT administrators and consumers alike are starting to think more about IP addresses. Here are the answers to a few questions about the numbers that make the Internet work.
1. Why do we need IP addresses?
Just as every letter sent through the postal service needs addresses to show both where it's going and where it's coming from, every packet on the Internet needs two IP addresses: destination and source. Those addresses direct packets to and from PCs, servers, and virtual machines. Each of those kinds of machines may have multiple IP addresses, some public and some just for use on the local network.
2. What do IP addresses do for me?
IP addresses are used for many Internet applications, some of them invisible to users, including machine-to-machine communications. Even in the simplest example, Web browsing, there are several steps. When you type in the URL for a website, your browser calls a piece of software on your device, the DNS (Domain Name System) resolver, and tells it to contact a name server, which may exist on your corporate LAN or at your ISP. The DNS resolver asks the name server to find an IP address that is assigned to the domain you typed in. If it finds one, it sends that address back to the browser, which uses the address to take you to the right domain.
3. How many IPv4 addresses are there?
There are about 4 billion IPv4 addresses, because the length of the addresses in binary form -- 32 bits -- allows for that many possible unique combinations. IPv6 addresses are 128 bits long, so there are many more unique sequences of numbers that can be created for them: specifically, 3.4 times 10 to the power of 38. This is also known as 340 undecillion. The word undecillion designates a number with 11 sets of three zeros, plus one more set in the numbering system used in the U.S. and many other countries. So the number is rendered as 340,000,000,000,000,000,000,000,000,000,000,000,000. Most experts believe that 340 undecillion addresses is essentially an inexhaustible supply.
4. Am I likely to get stuck without an IPv4 address?