Domain names and IP addresses

As we have seen, computers send information across the Internet to other computers (hosts). People refer to Internet hosts by their domain names, but computers address other computers using their Internet Protocol addresses (IP addresses). These are 32 bit numbers and are usually written as four decimal numbers. For example, the IP address of the computer you got this Web page from is 155.135.55.94.

IP addresses are easy for computers to handle, but they are awkward for people, so we use the domain name system (DNS). The format of the typical domain name is:

<host name>.<organization name>.<top-level domain name>

For example, som.csudh.edu, is a host in the csudh.edu domain. The domain "csudh.edu" belongs to California State University at Dominguez Hills, which is an educational organization. (There are many other hosts in the csudh.edu domain).

The domain name system is hierarchical, edu is a top-level domain, csudh.edu is a second-level domain, etc. In practice, most domain names are three or four levels, for example:

IP address Domain name Top-level
domain		 Second-level
domain		 Third-level
domain
155.135.55.94 bpastudio.csudh.edu edu csudh none
132.248.10.7 www.unam.edu.mx mx edu unam

As you see, www.unam.edu.mx, a host at the Universidad Nacional Autónoma de México, has a four-level domain name. It is registered in the mx top-level domain, indicating that it may be in Mexico.

Looking up IP addresses

When a user inputs a domain name, say as part of a URL, it must be converted to an IP address. This is done by computers programmed as domain name servers (also DNS). Every domain has at least one domain name server.

You can also manually look up information on domain names and IP addresses. You can do so using DOS commands that come with Windows or this tool:

Target IP Address or Host Name:
IP Lookup powered by ipchecking.com

There are also Web sites like kloth.net that do more detailed lookups for you.

You can use IP addresses as well as domain names in URLs. For example, you can go to my home page using http://155.135.55.94/fac/lpress/.

If you have a typical ISP account at home, you probably do not have a fixed IP address, but get a get new, dynamic IP address every time you connect to the Internet. (Your ISP will give you a fixed IP address for an extra monthly fee). Services like DynDNS and TZO allow you to pick a domain name and automatically associate it with your IP address whenever it changes.

For statistics on domain sizes and organizations that register domains, see the ISC domain survey and ICANN statistics.

Disclaimer: The views and opinions expressed on unofficial pages of California State University, Dominguez Hills faculty, staff or students are strictly those of the page authors. The content of these pages has not been reviewed or approved by California State University, Dominguez Hills.