Domain names

Looking up a web page

When you want to visit a web page you start up your favorite web browser and enter its address, such as www.tagbytag.org. We've learned in the previous chapter that every computer has an unique IP address. Thus, it should be possible to access a web page directly through its web server IP, right?

Let's test this with TagByTag's server IP: 69.65.104.19

1. Copy the IP into your web browser bar and hit enter.
2. You should now see the TagByTag homepage! This doesn't always work, though. It's up to the server administrator to decide what to do when the IP is accessed directly.

Domain names and URL

Because IP numbers are rather painful to remember, human-friendly domain names were introduced. They are part of an URL (uniform resource locator). A typical simple URL looks like this:

A simple URL

Let's have a look at the individual parts:

• Protocol: this tells the client and server in which way to communicate. HTTP (hypertext transfer protocol) is the standard for transferring information over the internet. FTP (file transfer protocol) is used to transfer larger files between computers.
• Subdomain: provides a way of organizing a site into separate parts (e.g. a visitor area and a member area). Subdomains can be located on the same server or even point to a completely different server. Although www is technically a subdomain it is usually used for accessing the main site (i.e. the domain).
• Domain: this is the actual domain name which is used for identifying a computer on the internet. Like the IP it must be unique. Domain names must have at least 2 letters before the dot and a valid ending after it (com, net, org, ...).
• Path: this tells the server in which folders to look for a certain file to be sent to the client.

And here follows a full URL with all the bells and whistles one could ask for:

A full URL

We won't go into the details, though, as this is better suited for an advanced tutorial. The 3 lines are just for space reasons, think of it as one long link.

DNS

From your web browser to the DNS

All that's left now is something to turn those domain names into IP numbers. This is where the DNS (domain name system) kicks in. The DNS works similar to a phone book. For example if you want to know the IP number of tagbytag.org the DNS can be used to tell you it's 69.65.104.19.

Instead of phone books we have computers, so-called name servers or DNS servers, that respond to DNS queries.

For most home users this DNS server is provided by their ISP. If your ISP's DNS server doesn't have the domain name / IP you're looking for it will ask other DNS servers on the internet until it succeeds or fails (such as when you entered a non-existent domain name).