10 things to know about the move to IPv6

The depletion of IPv4 addresses is turning the spotlight to the Internet's numbering system

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?

If you are a mobile Internet subscriber in Asia, you may pretty soon, according to Cricket Liu, vice president of architecture at Internet software vendor InfoBlox. APNIC (Asia-Pacific Network Information Centre) kicked off the final allocation of IPv4 addresses to the regional registries because it needed two new blocks of addresses to satisfy fast-growing demand in the region.

Mobile phones are the first Internet devices for many people in developing parts of Asia, and assigning IP addresses to those is part of the reason for the crunch. When mobile operators run out of IPv4 addresses, they will have to resort to selling phones with only an IPv6 address, Liu said.

5. What would happen if I only had IPv6 and tried to go to an IPv4-only website?

In the worst case, you wouldn't even get the familiar "server not found" message, according to Liu. This type of warning, also known as a "404" message, comes from the Web server of an existing domain that no longer hosts the page you're looking for. Without a usable IP address, no Web server can be reached. "It would probably just give you some sort of a network error," Liu said. Leo Vegoda, manager of number resources at the ICANN, said the message you receive will be up to your browser vendor.

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