The best way to explain the differences between these different items—that are related but different—is to break it down by example. Think of networks like a tree:
- There is a trunk that separates into major branches.
- Branches which in turn separate into smaller branches.
- Smaller branches eventually leading to individual leaves.
The “trunk” and “branches” are “domains” and “subdomains” and the leaves are individual devices, like computers. So let’s start there.
A “computer name” is strictly a local convention: I have a computer named
giacomo1968. This computer name is simply the name I have assigned my local machine. Nobody outside of my LAN will know this computer name; this is strictly a local setting.
A “hostname” (aka “nodename”) is a network identifier: If I wanted to publicly advertise my local computer to others, I would have to attach a “hostname” to the IP address of my computer. The “hostname” doesn’t really have to have anything to do with the computer name, but many times administrators like to use the same name to make things easier to understand. Also a “hostname” doesn’t always mean the computer is exposed to the Internet; it’s just an easy way to let others on your network where/what your computer is. As Wikipedia explains; emphasis is mine:
In computer networking, a hostname (archaically nodename) is a
label that is assigned to a device connected to a computer network and
that is used to identify the device in various forms of electronic
communication such as the World Wide Web, e-mail or Usenet.
- A fully qualified domain name (FQDN) is just that; a fully qualified domain name: Now that might seem confusing but you need to think of it this way: It just means in the great scheme of things, what is the actual “path” to get to a computer. So let’s say I have my computer named
giacomo1968 and it has a hostname on my LAN that is
giacomo1968. Within the context of my LAN, that
giacomo1968 is my computer name, and is my “hostname” and can possibly be considered my “fully qualified domain name” depending on my LAN architecture. Meaning let’s say my office LAN has other nodes in it like,
second_floor and my “hostname” of
giacomo1968 is on the
second_floor network. Well, if that were the case, then my LAN-based “fully qualified domain name” would be
giacomo1968.second_floor and that is it. But let’s say I worked at a big company named “big_company.com” and my computer were somehow exposed to the world. Then in that case, my WAN-based “fully qualified domain name” could be
giacomo1968.second_floor.big_company.com or maybe even just
giacomo1968.big_company.com if the network administrator didn’t want to be hassled with the “second_floor” designation. Again, as Wikipedia explains; emphasis is mine:
A fully qualified domain name (FQDN), sometimes also referred to as an
absolute domain name, is a domain name that specifies its exact
location in the tree hierarchy of the Domain Name System (DNS). It
specifies all domain levels, including the top-level domain and the
root zone. A fully qualified domain name is distinguished by its lack
of ambiguity: it can be interpreted only in one way.
The “www” in some website names is a “hostname” designation that is historical in nature: Basically, back in the 1990s when the world wide web was still in it’s infancy, networks had many different services attached to them. And mainly in an academic context. So there would be a place like
whattsamattau.edu and that school would have FTP services on
ftp.whattsamattau.edu, email on
mail.whattsamattau.edu and so on… So when the world wide web came along, they would just have placed the web server on
www.whattsamattau.edu. Nowadays, everyone—and seemingly everything in the world—has a website. And many people just register domains just for the web service. So the historic convention of
www is discarded in many cases. Many people have websites without
www but will still have accommodations to redirect
www traffic to the main, non-
www hostname. But technically speaking,
www can still be considered a hostname.
BONUS (Never Asked But Mentioned/Implied): What is a “subdomain” in the great scheme of things: A subdomain is basically just a child of a parent domain/hostname. So in my example of
giacomo1968 can be considered a subdomain of
big_company in that can be considered to be the “domain” with
.com being the top-level domain (TLD). Yet again, as Wikipedia explains; emphasis is mine:
In the Domain Name System (DNS) hierarchy, a subdomain is a domain
that is part of a larger domain.
Now after drafting all of this, it can be confusing. Many computer names are hostnames are fully-qualified domain names and in some cases they could be subdomains. It’s all a matter of context. And looking at articles online the words “domain,” “host” and “node” are used fairly loose and fast all over the place. So in my opinion, many uses of these terms are synonymous.
Which also helps to explain your confusion in item number 2:
If “mail” in
mail.google.com and “developers” in
developers.google.com are called subdomains why is
en.wikipedia.org called hostname? What is the difference
between host name and subdomain?
en.wikipedia.org is a subdomain and a hostname. The
en in the
en.wikipedia.org is a subdomain of the domain name
en.wikipedia.org is in itself a hostname since
en.wikipedia.org has an IP address connected to it and thus a computer connected to that IP address as well. Meaning the
en itself is semantically considered a subdomain, but it is also a hostname because there is a host (computer) connected to the IP address connected to
en.wikipedia.org. So knowing that
developers.google.com are hostnames as well which are both subdomains of
To possibly make things clearer think of it like this; let’s use the non-existant subdomain
fakename.wikipedia.org as an example:
fakename.wikipedia.org is a top-level domain (TLD). As a top-level domain it does not resolve to an IP address. Thus it is not a hostname. It is simply a naming convention.
fakename.wikipedia.org is a domain name and it has the potential to be a hostname as well if it is connected to an IP address and resolves to a computer when one goes to
fakename.wikipedia.org is a subdomain name since it is a child domain of
wikipedia.org in the context of the domain name itself. It can be a hostname if it connected to an IP address and resolves to a computer when one goes to
If you ping
fakename.wikipedia.org you are attempting to ping the
fakename subdomain of
wikipedia.org. If that ping dies — which it most likely will since it is fake — that means that the host is down or it does not exist. If it dies with an IP address connected to it, that would mean the hostname
fakename.wikipedia.org is down. If the ping dies with 100% no IP address connected to it, them it would mean that the subdomain
fakename.wikipedia.org is an invalid hostname.
Yes, this can all be confusing. But what it boils down to is the difference between what a domain/subdomain is in the context of DNS entries versus what domain/subdomain is in the context of having a host/computer connected to it.
So if this all started with your curiosity about
www, that is a historic hostname/nodename/subdomain that is not really used as a convention as much anymore but is still so commonly used that many sites have some accommodations in place to “catch” requests to
www and redirect them to the main hostname of a website.