Java Reference
In-Depth Information
cessing Information,” in the
Psychological Review
, Vol. 63, pp. 81-97. This is why phone
numbers are broken into three- and four-digit pieces with three-digit area codes. Ob‐
viously, an IP address, which can have as many as 12 decimal digits, is beyond the
capacity of most humans to remember. I can remember about two IP addresses, and
then only if I use both daily and the second is on the same subnet as the first.
To avoid the need to carry around Rolodexes full of IP addresses, the Internet's designers
invented the Domain Name System (DNS). DNS associates hostnames that humans can
remember (such as
login.ibiblio.org
) with IP addresses that computers can remember
(such as
152.19.134.132
). Servers usually have at least one hostname. Clients often have
a hostname, but often don't, especially if their IP address is dynamically assigned at
startup.
Colloquially, people often use “Internet address” to mean a host‐
name (or even an email address, or full URL). In a topic about net‐
work programming, it is crucial to be precise about addresses and
hostnames. In this topic, an address is always a numeric IP address,
never a human-readable hostname.
Some machines have multiple names. For instance,
www.beand.com
and
xom.nu
are
really the same Linux box. The name
www.beand.com
really refers to a website rather
than a particular machine. In the past, when this website moved from one machine to
another, the name was reassigned to the new machine so it always pointed to the site's
current server. This way, URLs around the Web don't need to be updated just because
the site has moved to a new host. Some common names like
www
and
news
are often
aliases for the machines providing those services. For example,
news.speakeasy.net
is an
alias for my ISP's news server. Because the server may change over time, the alias can
move with the service.
On occasion, one name maps to multiple IP addresses. It is then the responsibility of
the DNS server to randomly choose machines to respond to each request. This feature
is most frequently used for very high-traffic websites, where it splits the load across
multiple systems. For instance,
www.oreilly.com
is actually two machines, one at
208.201.239.100 and one at 208.201.239.101.
Every computer connected to the Internet should have access to a machine called a
domain name server
, generally a Unix box running special DNS software that knows
the mappings between different hostnames and IP addresses. Most domain name
servers only know the addresses of the hosts on their local network, plus the addresses
of a few domain name servers at other sites. If a client asks for the address of a machine
outside the local domain, the local domain name server asks a domain name server at
the remote location and relays the answer to the requester.
