Everybody
who uses the Web uses URLs (Uniform Resource Locators*), the addresses
that tell a browser where on the Web to go. And everybody who uses the
Web or e-mail uses domain names, which form a part of URLs and e-mail
addresses. However, many people are unaware of the full structure of these
things. Learning more about how URLs and domain names are structured will
help you understand the addresses you use every day. That knowledge will
also let you make thoroughly informed decisions about what addresses to
use for the sites you create, Mensa-related and otherwise.
To illustrate URL structure, here's a sample address within the local group site of which I'm the Webmaster:
http://palmbeach.us.mensa.org/calendar/200405cl.php
The first thing in the URL is the protocol or scheme, "http" (Hypertext Transfer Protocol), the one most commonly used on the Web. (Other schemes sometimes encountered include "https" for secure encrypted sites and "ftp" for file download sites using the File Transfer Protocol.) A colon separates the protocol from the remainder of the URL. Then the two slashes signal that what follows is a network server address, which is usually a domain name — though a numeric IP (Internet Protocol) address can also be used. Another slash marks the end of the server address, and what follows is a path indicating which document within the site is being accessed; if no path follows the domain name, then the site's main home page will be served.
Domain names are also found in e-mail addresses, which are of the form user@domain, with a username separated from the domain name by an "at" sign. When used as URLs, e-mail addresses are preceded by mailto: no slashes or "www."!
Thus, the URL as a whole is read left to right, as English-language text generally is. However, the domain name portion of the URL is a little different; it is read right to left, with dots separating segments in a hierarchical structure. Consequently, to understand palmbeach.us.mensa.org, you need to start at the rightmost element.
.org (pronounced "dot org") is the top-level domain
(TLD) in which the site is found. It is one of several TLDs that represent
different categories of entities on the Internet. .org domains
are intended for noncommercial organizations, the category in which Mensa
fits. .com domains are intended for commercial entities, so it
makes no logical sense for a not-for-profit group to use one, although
out of ignorance of domain name 
structure,
some do anyway. Other TLDs include .net for network providers,
.edu for educational institutions, .gov for government sites,
.mil for military sites and .int for international treaty
organizations. Some of these are worldwide in scope, while others, for
historical reasons, are limited to the United States, even though it would
have been more logical for country-specific sites to use country code
domains (described below). Since 2001, some new TLDs have been introduced:
.info for informational sites, .biz for businesses, .name
for individuals, .museum for museums, .aero for aviation-related
sites, .coop for co-ops and .pro for doctors, lawyers and
CPAs. Applications are pending for still more, such as .travel
for the travel industry.
Some of these domains have registration restrictions enforced by the registries or sponsoring organizations responsible for their management; for instance, you need to demonstrate that you're a legitimate educational institution to get a .edu domain. Others are wide open for anybody to register them, whether logically or illogically.
Each country in the world also has a country code domain, a two-letter domain used for sites wishing to identify themselves specifically as being associated with that country. .us is the country code domain for the United States, but it gets little use because Americans prefer generic domains; we Americans tend to act as if we own all the TLDs, just because the Internet was invented here. .uk is the domain for the United Kingdom, .ca for Canada, and so on. (.tv is the country code domain for the small island of Tuvalu, but they're trying to make some money by marketing it as if it really meant "television.") Some country code domains are subdivided at the second level, to represent different types of entities, just like the generic TLDs; .co.uk is for commercial sites in the U.K., while .org.uk is for U.K. organizations. Others, including .us, take registrations directly at the second level, though this is a fairly new development; until recently, you had to register under city and state subdomains, like miami.fl.us.
If you enter mensa.org into your browser, it will take you to the international Mensa home page. www.mensa.org works the same way. It's traditional to use www as the hostname to the left of a domain name when a website is being addressed, though most sites are set up to work with or without it. The actual full URL of the site has http:// before it, but browsers assume this and fill it in if you type the "bare" domain name. (However, when making links within HTML documents you are creating, it's important touse the full, correct syntax, protocol and all, or the URL may be misinterpreted.)
Many people and organizations don't go any further than this sort of
use of a domain
name to address a single website, but there is much more power to the
structure of domains. Once you control a domain name at any level of the
system, you can create an unlimited amount of structure beneath it, and
Mensa makes use of this. Hence, you can find the American Mensa site at
us.mensa.org, a subdomain of mensa.org using the country
code us as the third-level label. Note that this is not
within the .us country code domain; it's still within the .org
domain. Mensa (international or national) didn't have to pay an additional
registration fee to get this domain; Mensa could create it along with
other subdomains like au.mensa.org, for Australian Mensa, simply
by adding an entry to the host table in the configuration of the international
Mensa server. Also, the sites of American and Australian Mensa don't have
to be hosted on the same server as one another and international Mensa's,
since a subdomain can be "delegated" to point at a different
server anywhere in the world.
Subdomains don't have to stop at the third level; once a third-level domain name is delegated, the destination server can create further levels of domains beneath it. American Mensa uses this power to create logically addressed sites for its local chapters (e.g., palmbeach.us.mensa.org) and other functions (ag.us.mensa.org). Any local chapter can get its subdomain address from national Mensa at no charge, to use either for a site hosted on the national server (the hosting is also at no charge) or to point to an externally hosted site.
This system provides a consistent, logical, predictable, and economical way of giving addresses to all Mensa-related sites. However, as Mensans are cantankerously individualistic and resistant to central authority, Mensa sites aren't forced to use these addresses, and some have chosen not to do so. I'm all for resisting authority, myself, but because I'm also in favor of logic, I urge Mensa Webmasters to choose consistency in addressing, to avoid the crazy quilt that results when everybody goes his own way.
One type of inconsistent addressing used by some national groups is to get a domain in their own country code — mensa.org.uk instead of uk.mensa.org, for instance. There is a sort of logic to this; by using such a domain, you're unambiguously indicating that the site is in a given country, which to some people (particularly fellow countrymen) may be of greater importance than indicating that the site is part of the global structure of Mensa.
Of less logic is to get a separate .org domain for a national or local group; this forsakes any opportunity to use domain structure to indicate either its kinship with the rest of Mensa or with its home country. Even worse (but, unfortunately, adopted by some groups) is to use a .com domain (or the equivalent within a country code, like .com.br in Brazil), inappropriately implying commerciality.
One offender is the American Mensa National Office, which itself turned
its back on the logical naming hierarchy used by the rest of American
Mensa officialdom and put its 
staff
e-mail addresses in the unnecessary domain name americanmensa.org.
But they're not entirely consistent about it; the Bulletin editor
is a staff position, but still has a us.mensa.org address. And
the MERF director apparently couldn't decide whether to use director@foundation.us.mensa.org
or foundation@americanmensa.org, so both addresses
are shown in the Directory. Over the last few years, Mensa has made great
strides in getting the e-mail and Web addresses shown in the Bulletin
for various American Mensa officers, staff and functions to line up logically
and consistently, but the staff ones, with their different domain, stand
out like a sore thumb.
As is the case with any language, a non-native speaker who learns and uses the proper words and grammar will be better understood. This particular "language" is logical, and is rapidly becoming a worldwide tongue. Those who wish to keep up with communication in the 21st century will need to learn to converse in a way that is intelligible. As The High I.Q. Society, Mensa and its local groups should be in the vanguard.
*Actually, the "techie geeks" prefer to call them URIs (Uniform Resource Identifiers) these days, on the theory that it's sometimes desirable to have a way to identify an object even if you don't know where to locate it (e.g., when you're asking if anybody's seen the car keys you misplaced). Recent projects, such as the "Semantic Web," which aim to map the relationships among entities (both ones with known locations and ones without) actually do need identifiers that don't have a specific location embedded in them, and URI schemes have been developed for that purpose. However, the term "URL" has entered the popular culture, so it will probably never be displaced even if the new term is more technically accurate.
— Dan Tobias
Dan's Mail Format Site
Dan's Web Tips
Dan's Domain Site