The DNS: The Internet’s Phonebook

You can actually type an IP address of a Web site into a Web browser and that page will show up. But that doesn’t help users much because four sets of numbers are really hard to remember.

This is where the domain name service (DNS) comes in. The domain name service is a distributed database that looks up the host and domain names that you enter and returns the actual IP address for the computer that you want to communicate with. It’s like a big, hierarchical set of phone books capable of finding Web servers, e-mail servers, and more. These “phone books” are called nameservers —and when they work together to create the DNS, they can get you anywhere you need to go online.

Figure 12.3

When your computer needs to find the IP address for a host or domain name, it sends a message to a DNS resolver, which looks up the IP address starting at the root nameserver. Once the lookup has taken place, that IP address can be saved in a holding space called a cache, to speed future lookups.

To get a sense of how the DNS works, let’s imagine that you type www.yahoo.com into a Web browser. Your computer doesn’t know where to find that address, but when your computer connected to the network, it learned where to find a service on the network called a DNS resolver. The DNS resolver can look up host/domain name combinations to find the matching IP address using the “phone book” that is the DNS. The resolver doesn’t know everything, but it does know where to start a lookup that will eventually give you the address you’re looking for. If this is the first time anyone on that network has tried to find “www.yahoo.com, ” the resolver will contact one of thirteen identical root nameservers. The root acts as a lookup starting place. It doesn’t have one big list, but it can point you to a nameserver for the next level, which would be one of the “.com” nameservers in our example. The “.com” nameserver can then find one of the yahoo.com nameservers. The yahoo.com nameserver can respond to the resolver with the IP address for www.yahoo.com, and the resolver passes that information back to your computer. Once your computer knows Yahoo! ’s IP address, it’s then ready to communicate directly with www.yahoo.com. The yahoo.com nameserver includes IP addresses for all Yahoo! ’s public sites: www.yahoo.com, games.yahoo.com, sports.yahoo.com, finance.yahoo.com, and so on.

The system also remembers what it’s done so the next time you need the IP address of a host you’ve already looked up, your computer can pull this out of a storage space called a cache, avoiding all those nameserver visits. Caches are periodically cleared and refreshed to ensure that data referenced via the DNS stays accurate.

Distributing IP address lookups this way makes sense. It avoids having one huge, hard-to-maintain, and ever-changing list. Firms add and remove hosts on their own networks just by updating entries in their nameserver. And it allows host IP addresses to change easily, too. Moving your Web server off-site to a hosting provider? Just update your nameserver with the new IP address at the hosting provider, and the world will invisibly find that new IP address on the new network by using the same old, familiar host/domain name combination. The DNS is also fault-tolerant—meaning that if one nameserver goes down, the rest of the service can function. There are exact copies at each level, and the system is smart enough to move on to another nameserver if its first choice isn’t responding.