{"id":3590,"date":"2026-10-07T14:27:26","date_gmt":"2026-10-07T14:27:26","guid":{"rendered":"https:\/\/www.ucartz.com\/blog\/what-is-fqdn-fully-qualified-domain-name\/"},"modified":"2026-10-07T14:27:34","modified_gmt":"2026-10-07T14:27:34","slug":"what-is-fqdn-fully-qualified-domain-name","status":"publish","type":"post","link":"https:\/\/www.ucartz.com\/blog\/what-is-fqdn-fully-qualified-domain-name\/","title":{"rendered":"What Is an FQDN? Fully Qualified Domain Names Explained"},"content":{"rendered":"\n

Short answer:<\/strong> a fully qualified domain name (FQDN) is the complete name of one host on the internet, with every part spelled out from the host up to the top-level domain, for example mail.example.com<\/code>. Strictly speaking it ends with a dot, mail.example.com.<\/code>, where the final dot stands for the root of the DNS. In everyday use the dot is left off. For server owners the FQDN matters in four places: the server’s hostname, the name a mail server introduces itself with, SSL certificates and DNS zone files, where a missing dot is a classic mistake.<\/p>\n\n\n\n

\"Terminal
With 127.0.1.1 web1.example.com web1 in \/etc\/hosts, hostname -f prints the FQDN and hostname -d the domain.<\/figcaption><\/figure>\n\n\n\n

The parts of an FQDN<\/h2>\n\n\n\n
Part<\/th>In mail.example.com.<\/code><\/th>What it is<\/th><\/tr><\/thead>
Host label<\/td>mail<\/code><\/td>The individual machine or service<\/td><\/tr>
Domain<\/td>example<\/code><\/td>The name you registered<\/td><\/tr>
Top-level domain<\/td>com<\/code><\/td>The TLD, such as .com, .in or .org<\/td><\/tr>
Root<\/td>the final .<\/code><\/td>The top of the DNS tree, usually not written<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Each part between the dots is called a label. RFC 9499<\/a>, the IETF’s DNS terminology document, explains the ambiguity: strictly, a fully qualified name includes the empty root label and is written www.example.net.<\/code>, but names written without the final dot are still called fully qualified. The same document sets the limits: each label can be up to 63 octets, and the whole name at most 255 octets in DNS wire format.<\/p>\n\n\n\n

The opposite of an FQDN is a partially qualified or relative name, such as plain mail<\/code>. It only means something in context, for example on a network whose search domain is example.com<\/code>.<\/p>\n\n\n\n

FQDN, hostname, domain name and URL<\/h2>\n\n\n\n
Term<\/th>Example<\/th>Meaning<\/th><\/tr><\/thead>
Short hostname<\/td>web1<\/code><\/td>The machine’s own name, without a domain<\/td><\/tr>
Domain name<\/td>example.com<\/code><\/td>The registered name; it can also be an FQDN when it points at a server itself<\/td><\/tr>
FQDN<\/td>web1.example.com<\/code><\/td>Host plus domain plus TLD: one exact host<\/td><\/tr>
URL<\/td>https:\/\/web1.example.com\/status<\/code><\/td>Protocol, FQDN and path: one exact resource<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The FQDN is the part of a URL between https:\/\/<\/code> and the first slash. If you are unsure how a subdomain fits in, our guide What Is a Subdomain?<\/a> covers it: web1.example.com<\/code> is a subdomain of example.com<\/code> and also the FQDN of the server called web1.<\/p>\n\n\n\n

Why the trailing dot matters<\/h2>\n\n\n\n

Most of the time you never see the root dot. It matters in two places.<\/p>\n\n\n\n

DNS zone files<\/h3>\n\n\n\n

RFC 1035<\/a> says names in a zone file that end with a dot are absolute; names without one are relative and get the zone’s origin added. So in the zone for example.com<\/code>, forgetting the dot changes the meaning. We loaded a test zone containing these records (SOA and NS records left out here) with BIND 9.20:<\/p>\n\n\n\n

$ORIGIN example.com.\nwww     IN CNAME example.com.\nshop    IN CNAME example.com\n@       IN MX  10 mail.example.com\nmail    IN A   203.0.113.30<\/code><\/pre>\n\n\n\n
named-checkzone<\/code> accepted the file and reported OK<\/code>, with only a warning about the mail server:<\/p>\n\n\n\n
zone example.com\/IN: example.com\/MX 'mail.example.com.example.com' has no address records (A or AAAA)<\/code><\/pre>\n\n\n\n
Printing the zone as BIND understood it showed the damage:<\/p>\n\n\n\n
example.com.        3600 IN MX     10 mail.example.com.example.com.\nshop.example.com.   3600 IN CNAME  example.com.example.com.\nwww.example.com.    3600 IN CNAME  example.com.<\/code><\/pre>\n\n\n\n
Only www<\/code>, written with the dot, is right. Mail and the shop would both break. Web-based DNS editors in control panels usually add the dot for you, but if you edit zone files by hand, write every full name with its final dot.<\/p>\n\n\n\n
Search domains<\/h3>\n\n\n\n
When \/etc\/resolv.conf<\/code> contains search example.com<\/code>, a short name like www<\/code> is tried as www.example.com<\/code>. In our test, getent hosts www<\/code> returned the addresses of www.example.com<\/code>. With a final dot, www.<\/code> is treated as complete and is not expanded, so dig +search www.<\/code> returned nothing. The dot is how you tell software “this name is already complete”.<\/p>\n\n\n\n
Setting your server’s FQDN on Linux<\/h2>\n\n\n\n
On a server, hostname<\/code> shows the short name and hostname -f<\/code> the FQDN. According to the Debian hostname manual<\/a>, the FQDN is the name the system’s resolver returns for the hostname, and the usual way to set it is a line in \/etc\/hosts<\/code> like this:<\/p>\n\n\n\n
127.0.1.1 web1.example.com web1<\/code><\/pre>\n\n\n\n
The order on that line is important. We tested both orders on Debian 13 with the hostname package, version 3.25:<\/p>\n\n\n\n
\/etc\/hosts<\/code> line<\/th>hostname -f<\/code><\/th>hostname -d<\/code><\/th><\/tr><\/thead>
127.0.1.1 web1 web1.example.com<\/code><\/td>web1<\/code><\/td>(empty)<\/td><\/tr>
127.0.1.1 web1.example.com web1<\/code><\/td>web1.example.com<\/code><\/td>example.com<\/code><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
With the short name first, the server does not know its own domain. That is harmless for a web server, but mail software and control panels read this value. For the full procedure, see how to set or change the hostname in Linux<\/a>.<\/p>\n\n\n\n
Where a correct FQDN matters<\/h2>\n\n\n\n