FQDN stands for Fully Qualified Domain Name and it represents the actual complete domain name of a system ( for example a computer or a server or some other device that can posses an IP address, which can also posses a DNS registration ) inside a Domain which specifies the exact location in the tree hierarchy of the DNS.
The FQDN contains two parts, the first part is the hostname – which identifies the device on the given domain and the second part: the domain name – which of course identifies the name of the domain.
We could have, for example, a host which is actually a computer named PC1 and a domain this computer belongs to…let’s call it compinfopro.com. The FQDN of this host would be PC1.compinfopro.com which also means that the host PC1 is unique inside this domain but not in the world. So you can have a large number of computers with the same hostname on the Internet but it would be unique for each domain and they would be unique on the Internet due to the FQDN, which identifies them.
In this example we also have the .com extension of the domain which is actually the Top Level Domain and this is like a root directory to a computer.In other words, this is where it all starts, every directory or folder originates from this location.
For example, within .com TLD we would have our compinfopro.com domain which would have the authority to create subdomains, that is if we really needed such a thing and the domain would be created. This also applies to web addresses where we have www.compinfopro.com – this being our FQDN on the web.
It is better if you use the FQDN when you are trying to connect to some remote system, like for example using SSH or Telnet to connect to a server. Not succeeding to do this, would cause the DNS server to fail in contacting the given hostname after checking its DNS registration table.
If you are using only the hostname and not the FQDN, you might get into different cases where you actually can’t connect to a given system due to the following possible causes:- the DNS suffix search order in the TCP/IP stack is not correct.- the DNS registration table is corrupted.
If this happens, using the FQDN will temporary correct your problem and the DNS server will be able to contact the given hostname.
If you are trying to connect to a hostname inside another domain, even if we are talking about web addresses, you should be trying to connect using the FQDN, as your local DNS server might not know the corresponding name-ip resolution of the given remote hostname.
I would also recommend reading the article about DNS suffixes on how to change order or how to add them. This next article would also help you in understanding what a DNS suffix stands for and where and why it would be wise to add another.