What is a fully qualified domain name (FQDN)?
A fully qualified domain name (FQDN) is the complete domain name for a specific computer, or host, on the Internet. The FQDN consists of two parts: the hostname and the domain name. For example, an FQDN for a hypothetical mail server might be mymail.somecollege.edu. The hostname is mymail, and the host is located within the domain somecollege.edu.
In this example, .edu is the top-level domain (TLD). This is similar to the root directory on a typical workstation, where all other directories (or folders) originate. (Within the .edu TLD, Indiana University Bloomington has been assigned the indiana.edu domain, and has authority to create subdomains within it.)
The same applies to web addresses. For example, www.indiana.edu is the FQDN on the web for IU. In this case, www is the name of the host in the indiana.edu domain.
When connecting to a host (using an SSH client, for example), you must specify the FQDN. The DNS server then resolves the hostname to its IP address by looking at its DNS table. The host is contacted and you receive a login prompt.
If you are using only the hostname (without the domain information) to connect to a server, the application you’re using may not be able to resolve the hostname. This can happen if either the DNS suffix search order in your computer’s TCP/IP properties is incorrect, or the DNS table is corrupted. In these cases, entering the host’s FQDN will allow DNS to locate the server. Also, if you are trying to connect to a remote host that is not local to your Internet service provider (ISP), you will probably have to use the FQDN. For example, it’s unlikely that a DNS server at IU would have a listing for remote hosts at another university or an unrelated ISP.