DNS, the internet's phone book

Routers move packets by number. People remember names. The Domain Name System, DNS, connects the two. You hand it a name like wikipedia.org and it hands back an IP address.

It's often called the internet's phone book, and that's close. The difference is that no single book exists. The directory is split across a huge number of servers, and each one is responsible for its own small part.

Read the name backwards

DNS reads names from right to left. Take en.wikipedia.org. The last part, org, is a top-level domain. Wikipedia registered its name under org. Then Wikipedia created en under its own name, for the English site. Each level hands control of the level below to someone else, which is how one system can cover every name on the internet without one owner.

A lookup, step by step

Your phone doesn't do the hard work itself. It asks a resolver, a server run by your internet provider or by a company you've chosen. If the resolver already has the answer saved, it replies at once. If not, it walks down the levels:

1. It asks a root server who handles org. The root server doesn't know Wikipedia's address, but it knows which servers are in charge of org.
2. It asks one of those org servers who handles wikipedia.org. They point it to Wikipedia's own name servers.
3. It asks Wikipedia's name servers for en.wikipedia.org and finally gets the address.

The resolver passes the address back to your phone and keeps a copy for next time.

Step through the lookup and watch every reply return to the resolver. Turn on the cached answer to skip the referrals.

There are only 13 root server names, run by about a dozen organizations. Each name is really a fleet of machines spread around the world that all answer to the same address, so your question reaches a nearby one.

Saved answers keep it fast

If every lookup went all the way to the root, the root servers would drown. Instead, every answer comes with a time to live, a number of seconds it may be reused. A popular name is almost always saved at your resolver already, so the full walk is rare.

The catch shows up when a site changes its address. Some people keep getting the old one until their saved copy expires.

The answer depends on who asks

Big sites keep copies of their content in data centers all over the world. Their name servers often reply with the address of a copy near you. Someone in Tokyo and someone in Toronto can type the same name, get different numbers, and both be right.

When DNS breaks

If DNS fails, the internet looks dead even though every cable works. Your connection is fine. Your phone just can't turn names into addresses. A surprising number of large outages turn out to be DNS problems, often a bad configuration change.