URL: Uniform Resource Locator
When you use the Web or send an e-mail message, you use a domain name to do it. For example, the Uniform Resource Locator (URL) "http://www.howstuffworks.com" contains the domain name howstuffworks.com. So does this e-mail address: firstname.lastname@example.org. Every time you use a domain name, you use the Internet's DNS servers to translate the human-readable domain name into the machine-readable IP address. Check out How Domain Name Servers Work for more in-depth information on DNS.
Top-level domain names, also called first-level domain names, include .COM, .ORG, .NET, .EDU and .GOV. Within every top-level domain there is a huge list of second-level domains. For example, in the .COM first-level domain there is:
Every name in the .COM top-level domain must be unique. The left-most word, like www, is the host name. It specifies the name of a specific machine (with a specific IP address) in a domain. A given domain can, potentially, contain millions of host names as long as they are all unique within that domain.
DNS servers accept requests from programs and other name servers to convert domain names into IP addresses. When a request comes in, the DNS server can do one of four things with it:
- It can answer the request with an IP address because it already knows the IP address for the requested domain.
- It can contact another DNS server and try to find the IP address for the name requested. It may have to do this multiple times.
- It can say, "I don't know the IP address for the domain you requested, but here's the IP address for a DNS server that knows more than I do."
- It can return an error message because the requested domain name is invalid or does not exist.