DNS Primer: hostname vs subdomain…

2010 05 18 One area of misconception about The Internet’s Domain Name System (DNS) is confusing “hostname” with “subdomain”.  This is very understandable, because in a typical 3-part domain “hostname” takes the position as “subdomain”. Yet it should be understood that hostname is NOT subdomain.

Let me explain…

A hostname is an actual server, website or web application.
A subdomain is merely an extended part of the domain itself.

This is best illustrated by examples:�
SAY you have your corporate website (typically-but-not-REQUIRED-to-be-called “www”) running on an actual server. This “www” is indeed a “hostname”, and if your company domain is “vogtland.ws”, then the entire browser address would be http://www.vogtland.ws . Since this address follow the pattern of “protocol + hostname + domain”, this is often referred to as a “fully qualified” address and the stuff to the right of the “//” is called the Fully Qualified Domain Name (FQDN). Just remember “hostname + domain = FQDN”, and you’ve got it.

MOST domains are made up only of 2 parts:
1. the “top-level”  or “first-level” domain – there are currently about 280 total
–   248 country domains (e.g. “.us”, “.ca”, “.uk”, “.ru”) and�
–   20 “general”  domains (e.g. “.com”, “.org”, “.net”) and
–  12 other domains used for testing & private purposes.

2. The 2nd-level domain -typically the company, university or organization name itself in an easy-to-remember format:
– “IIT” in iit.edu,    “homedepot” in homedepot.com  “wikipedia” in wikipedia.org

In these scenarios the word taking the 3rd position from the right would be an actual server or hostname:
www.iit.edu   or   www.homedepot.com   or www.wikipedia.org   or even sandbox.markvogtltd.us  

However, there are times when a large organization wishes to further subdivide itself, and this is when SUBdomains are used. Here are some examples of longer FQDNs which contain not only hostnames but also SUBdomains:
www.northamerica.ford.com ,   webmail.chemistry.iit.edu ,   sandbox.chicago.markvogtltd.us , and sandbox.minneapolis.markvogtltd.us. In each of these cases, the left-most word is the actual server or hostname, while the word just left of the 2-level domain is merely a SUBdomain.

1. hostnames are MACHINES (servers).
2. subdomain are LOCATIONS (not machines)
3. hostnames are NOT subdomains
4. in typical 3-part fully-qualified domain names, the (3rd) position is taken by the hostname, but this is ALSO the position for a SUBdomain, which is why everyone gets confused (even tech support people working at notable domain registries, which irks me to no end).

Now you’re one of those who is NOT confused.

Spread the word. 😉



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