hostname - name of an actual addressable thing on the internet
domain name - the name of a network
root domain - this is actually "." or empty. the top most thing
tld, gtld - this is thing under root domain which has an entry in the root servers - com, co.uk etc
fqdn - this is really fully qualified hostname. so the full name in a host
sub domain - refers to a network rather a hostname. example.foo.com is a sub domain iff it's a network. otherwise it would be a hostname. foo.com is a subdomain of com.
zone - a concept of DNS where entries under a sub domain are delegated to a nameserver.
Girish Ramakrishnan
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