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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.
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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.
Go to the previous, next section.