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I understand that a FQDN is the combination of the domain name and the machine's (host's) name. But can I have a FQDN where what the hostname would be, is something different? Can this cause problems?

Example:

Hostname - home

Domain Name - server.example

FQDN - www.server.example

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Sure, you can put whatever you want in your DNS configuration. It does not need to match the server name.

Depending your context, it may be a good idea to create Host (A) record with the server name and an alias (CNAME) record for www that points to the host record.

It all depends your context. With a server name of "home", you're probably not talking about some massive online service - sounds more like a small in-house network.

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A device's hostname and a DNS FQDN are not necessarily tied to one another.

Imagine for example that Jane has a server whose hostname is simply "janes-mac-pro"; upon which she runs a web service, an LDAP service, and a mail server. She's been given a single IP address and opens up ports 443, 646, and 587 for clients to connect.

In her case, she has the option to make multiple FQDNs referencing her single IP address. In Bind-style notation it would look like this:

ldap.server.example. 7200   IN   A   192.0.2.1
mail.server.example. 7200   IN   A   192.0.2.1
www.server.example. 7200   IN   A   192.0.2.1

All three FQDNs above point to the same IP address, which in Jane's case is plumbed onto her computer- but none of these is her machine's actual hostname. She could also put in another record referencing that name if she wished.

There are other DNS record types which reference ports and service names, however most people don't need to be familiar with them.

The most common way in which having a different hostname for the device than the "hostname" portion of the FQDN would cause problems is if the device is set up to dynamically update the DNS record using the hostname information. This however is usually manually configured, and not default behavior outside of local Windows networks.

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