Ok, computers can be referred to by either their FQDN or their
Internet address. How can one user be expected to remember them all?
They aren't. The Internet is designed so that one can use either
method. Since humans find it much more natural to deal with words
than numbers in most cases, the FQDN for each host is mapped to its
Internet number. Each domain is served by a computer within that
domain, which provides all of the necessary information to go from a
domain name to an IP address, and vice-versa. For example, when
someone refers to foosun.bar.com, the resolver knows that it should
ask the system foovax.bar.com about systems in bar.com. It asks what
Internet address foosun.bar.com has; if the name foosun.bar.com
really exists, foovax will send back its number. All of this
"magic" happens behind the scenes.
Rarely will a user have to remember the Internet number of a site
(although often you'll catch yourself remembering an apparently
obscure number, simply because you've accessed the system
frequently). However, you will remember a substantial number of
FQDNs. It will eventually reach a point when you are able to make a
reasonably accurate guess at what domain name a certain college,
university, or company might have, given just their name.