A fully qualified domain name or FQDN has four parts.
First is the protocol part which is like http and https. Protocol is a set of rules that websites follow while transferring data across the web. Text data follows ‘Hyper Text Transfer Protocol’ and that’s why all website have http:// at the first place. File Transfer Protocol or ftp is for transferring files and protocols like https and sftp are just http and ftp with added security of SSL.
Next is the hostname or nodename. It’s a human-readable nickname of the service used by a device connected to the computer network. So if it’s a web service for domain, it has to be www hostname. Now www has more of less become redundant and not many sites uses it in their domain.
Third part is the second level domain. In layman’s terms, it’s just the core name of a website that a business wishes to have.
And the last part is the top level domain or TLD. TLD can be understood as the category under which a website is registered. Most have .com which is for commercial sites. Similarly, .org is for organization, edu for educational sites and so on.
Currently there are more than 300 TLDs out of which 22 are general domains like .com, .net and .org, 270+ are country based domains like .in, .au and .uk, and some 12 are for testing purposes.
Protocol+hostname+domain make a Fully Qualified Domain Address. And the portion after the protocol ie ‘Hostname+Domain’ is called Fully Qualified Domain Name or FQDN. It’s called so because it has ideal structure to be identified in the DNS or domain name system. DNS is just a phonebook like system containing domain names and IPs of websites mapped against each other.
Subdomains: When an organization desires to divide its website into different sections for different operations, subdomains are required. They can be defined as the subparts of a domain.
Apple has its online store at store.apple.com which is a subdomain of apple.com. Similarly HP provides buying and selling service at shopping.hp.com, customer care at www8.hp.com.
Subdomains make longer FQDN’s. They’re like Protocol+hostname+subdomain+domain.