A Beginner's Guide to Effective Email

Domain Names

Kaitlin Duck Sherwood

How To Read A Domain

The domain name is the thing that comes after the at sign (@) in an email address, like aol.com or arc.nasa.gov. The domain names have different words, separated by periods, that indicate different levels of organization. The size of the organization increases as you go left to right. The domain arc.nasa.gov, for example, is for Ames Research Center, which is part of NASA, which is one of many U.S. government entities.

If I wave my hands and simplify just a little bit, the left-most word is the name of the actual computer that handles the mail. Small organizations might only have one computer that does everything; larger organizations might have multiple computers. For example, Ames Research Center email addresses currently all go through mail.arc.nasa.gov.

If you get email from someone, and there is no at sign (@), then that probably means they have the exact same domain as you. For example, if pat@bogusname.com sends email to chris@bogusname.com, Chris might see only pat in the return address field.

If there seems to be something missing from the domain name, then your correspondent may share some domain information. For example, if pat@uno.flossrecycling.com sends email to chris@dos.flossrecycling.com, Chris might see only pat@uno in the return address field.

Common Three-Letter Top-Level Domains

The last word in a domain (also called the top-level domain) gives a clue to your affiliation. The three-letter Top-Level Domains (TLDs) have these meanings in theory.

TLD Meaning Examples
.com U.S. commercial business, a company ibm.com, att.com, ford.com
.net Network provider, Internet Service Provider webtv.net
.gov U.S. governmental agency whitehouse.gov, nasa.gov
.edu U.S. educational institution uiuc.edu, stanford.edu
.org Non-profit institution redcross.org, sfopera.org
.mil U.S. military army.mil
.int International itu.int
.biz Businesses

Once upon a time, all the above three-letter TLDs (aside from .int) were exclusively U.S. domains. (The Internet descends from the ARPANET, which was developed in the U.S.) They are still heavily U.S.-centric.

Two-Letter Top-Level Domains

If there is a two-letter top-level domain, that is a country code. Here are some examples:

TLD Country Examples
us United States city.palo-alto.ca.us, washington.k12.ia.us
uk United Kingdom cam.ac.uk, tvr.co.uk
my Malaysia parlimen.gov.my, jaring.my
de Germany (Deutschland) sgi.de
jp Japan www.hitachi.co.jp, www.nihon-u.ac.jp
to Tonga netsurf.to
tv Tuvalu internet.tv

An exhaustive list of country codes is at http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2.

Further Clues

Countries, especially the ones that are well-connected to the Internet, frequently have some meaningful structure in the next-to-last word in their domains. For example, ac is short for "academic" in the United Kingdom, New Zealand, and Japan. Oxford University, for example, is ox.ac.uk.

Universities in Europe frequently have the word "uni" in their domains somewhere, short for "university". Australia uses edu for its universities, as in usyd.edu.au.

Commercial entities frequently have co in the next-to-last word in their domain. For example, Hitachi Japan is at hitachi.co.jp.

United States two-letter domains usually have the two-letter state or territory abbreviation right before the .us. For example, the city of Palo Alto in California has the domain city.palo-alto.ca.us. (Remember that the scope increases as you go left to right: palo-alto.us.ca would be in Canada if it existed!) U.S. State and territory codes can be found at http://en.wikipedia.org/wiki/ISO_3166-2:US.

Children's schools in the United States frequently have the word k12 in them. (This is short for "Kindergarten through 12th grade", the U.S. terms for schools for students nominally ages 5-18.) Community colleges frequently have cc as the next-to-last word in the domain.

Canada also sometimes uses province codes; those can be found at http://en.wikipedia.org/wiki/ISO_3166-2:CA.

The UK also uses

(Note that unlike in the US, "school" in the UK means ONLY pre-university institutions, what in the US would be called K-12.)

France uses

There are a bunch of subdomains that have been defined in the us domain, including:

These rules are not always followed. For example, the State of California uses ca.gov when it really should use state.ca.us. Also, some of these categories are extremely rare. I've never seen a dni or a tec, for example.

Now Wait A Minute!

You may have noticed that some sites that don't seem to match their extensions. The domain internet.tv is in Canada, not in Tuvalu. Why does America On-Line use aol.com instead of aol.net? Why is netsurf.to in the United States?

Basically, money. The countries of Tuvalu and Tonga have raised badly-needed cash by selling the rights to their extensions to outside parties, who then sell them to other bidders. (They think that the English word "to" and the common abbreviation "TV" for "television" are worth something as extensions.) And since .com is what people try first when looking for a company, many entities chose to use that instead of something in their country's two-letter top-level domain.

There are also some classes of organization that don't fit any of the domains particularly well. Individuals who want to put up a web page are not companies, nor non-profits, nor military. For-profit arts organizations don't fit comfortably in either .com or .org.

Take the older domain names with a grain of salt. Most TLDs are "open", meaning that anybody can register a domain in that TLD without proving that they are in the "proper" group for that TLD. When people think it will get them some advantage, it happens. Restricted domains are one approach to make domain names more meaningful.

sponsored top-level domains. Sponsored TLDs

In an effort to help people be more confident about the legitimacy of Web sites, there are now TLDs that are run by a sponsoring organization that vets applications to restrict membership to legitimate members of the sponsoring organizations' community. For example, the .museum domain is run by the Museum Domain Management Association and only allows museums to register in the .museum TLD.

Sponsored TLDs

TLD Type of organization
.museum Museum
.aero Air transport industry
.coop Cooperatives
.pro Professionals (doctors, lawyers, and certified public accountants)
.travel Travel industry companies
.jobs Employment-related sites
.mobi Sites geared for mobile devices
.tel Voice-over-IP sites

Go on to Bibliography

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Created 22 Oct 1998
Modified to add more .us subdomains Dec 1999
Minor clarification 24 Feb 2000
Moved bibliography to be the last appendix, beautified page 23 May 2001
Added some information from Wikipedia's TLD page 23 June 2007

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