Difference between Host Name and Domain Name

There is often confusion about what is a host name and what is a domain name.

A domain name is the name that is purchased from a registrar. It will be something like fornesia.com. Note that there is no www at the beginning of a domain name. A domain name can be subdivided into sub-domains - for example www.fornesia.com. Once you own a domain, there is no reasonable limit to the number (or names) of the sub-domains you can create. A sub-domain can be allocated to a host machine - for example a PC. In fact many sub-domains can be allocated to the same host machine. The way sub-domains are allocated to a host machine is to create a record in the DNS (Domain Name Servers) that records the sub-domain name and the IP address of the host machine. Any requests for a sub-domain (e.g. www.fornesia.com) are converted to an IP address by DNS and the IP address is used to route the request through the network until it reaches the host machine.

In the early years of the Internet, each sub-domain would have a unique IP address so it was common for a host machine to have only one sub domain name. Nowadays, the common practice is to have many sub-domains with the same IP address. It is also common for the domain name to be converted to the IP address of the host machine that runs the www sub domain.

A host name is the unique name of a machine. When the host operating system is set up it is given a name. This name may reflect the prime use of the machine. For example, a host machine that converts host names to IP addresses using DNS may be called dns.fornesia.com and a host machine that is a web server may be called www.fornesia.com. When we need to find the host name from an IP address we send a request to the host using its IP address. The host will respond with its host name.

Another example from superuser website, hostname is the name given to the end-point (the machine in question) and will be used to identify it over DNS if that is configured domain is the name given to the 'network' it will be required to reach the network from an external point (like the Internet) It is usually written in the form, hostname.domain.com -- for example.
If you are in (say) a college campus named called 'The-University', and its domain is called 'theuniversity.org', a machine on the campus network called 'mymachine' would be addressed as, 'mymachine.theuniversity.org'.

Internet hostnames

From Wikipedia, Internet hostnames may have appended the name of a Domain Name System (DNS) domain, separated from the host-specific label by a period ("dot"). In the latter form, a hostname is also called a domain name. If the domain name is completely specified, including a top-level domain of the Internet, then the hostname is said to be a fully qualified domain name (FQDN). Hostnames that include DNS domains are often store

In the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, en.wikipedia.org consists of a local hostname (en) and the domain name wikipedia.org. This kind of hostname is translated into an IP address via the local hosts file, or the Domain Name System (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.
Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. So, for example, both en.wikipedia.org and wikipedia.org are hostnames because they both have IP addresses assigned to them. A hostname may be a domain name, if it is properly organized into the domain name system. A domain name may be a hostname if it has been assigned to an Internet host and associated with the host's IP address.