What is a hostname versus a computer name versus a subdomain versus www?

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, first_floor and 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.big_company.com the giacomo1968 can be considered a subdomain of big_company.com and 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 in en.wikipedia.org called hostname? What is the difference between host name and subdomain?

The en.wikipedia.org is a subdomain and a hostname. The en in the en.wikipedia.org is a subdomain of the domain name wikipedia.org. And 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 mail.google.com and developers.google.com are hostnames as well which are both subdomains of google.com.

To possibly make things clearer think of it like this; let’s use the non-existant subdomain fakename.wikipedia.org as an example:

• The org in 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.
• The wikipedia.org in 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 wikipedia.org.
• The fakename in 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 fakename.wikipedia.org.

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.

1. Hostname is an attribute of a system stored locally on that system. "Computer name" is what Windows uses to refer to the hostname.

2. A subdomain is a DNS concept. In DNS, domain names (domains for short) can be authoritative or non-authoritative - if they are non-authoritative, that means another server "handles" that domain. So in a domain such as www.mysite.invalid - one thing that could happen is that the root domain refers a lookup to a server handling the invalid domain, which then further refers the lookup to a server handling the mysite domain, etc.

   Your system might keep track of the domain it thinks it's on as well as a locally managed hostname.

3. You can change your computer's local hostname, but this won't automatically update any DNS server pointed to your machine unless you have software specifically doing that (such as Active Directory in a corporate Windows environment).

   Many DHCP clients are also capable of changing a computer's local hostname in response to a DHCP response from a DHCP server.

   Typically a system's internal hostname and external DNS name will match, but they don't have to, though 99% of time you want them to.

   You can set your computer's hostname to the same as its full external DNS, and it usually makes sense to do so.

4. Let's say you run a network somewhere, and have a DNS domain for it, such as mynetwork.invalid. Let's also say you are running your own DNS server, so any DNS requests for anything ending in mynetwork.invalid come to your DNS server. Now, let's say you have a couple servers, such as FTP, a Web server, etc. You'll have to create a record on your DNS server for each machine, and www was more a less a standard for Web servers in a network. So then ftp.mynetwork.invalid would point to your FTP server, and www.mynetwork.invalid would point to your Web server, etc.

   You might choose to have the plain old mynetwork.invalid pointing to the most common service you run, such as the Web server, to save people from typing a lot and make your URLs look better, but if someone enters simply mynetwork.invalid on an FTP client, there will be problems.

   However, most modern sites are only running one service, not a bunch of servers running a bunch of different servers. So then a shortcut is taken - mynetwork.invalid is the only one used, and you might create a second DNS entry www.mynetwork.invalid pointing to the same IP just for compatibility or those who are used to typing it.