Information Technology Reference
In-Depth Information
an Internet authority, as I will describe shortly; various Internet au
thorities maintain tables of domain names and IP numbers, and
other authorities store tables of IP numbers and Ethernet addresses.
In considering these tables, the Internet is now so large that it is not
practical for any one machine to record all possible addresses.
Instead, central machines are designated as authorities for various
parts of the network, and a local machine may or may not record
the few domain names, IP numbers, and Ethernet addresses it actu
ally uses. Any machine attempting to contact another machine must
first contact a central computer in charge of keeping track of the
necessary IP addresses so that it has access to that information.
To see how this type of lookup is done, suppose the local ma
chine (steenrod.cs.grinnell.edu) needs to determine the IP number
for cs.utexas.edu. When the machine is first turned on, it has no in
formation concerning other IP numbers, but its initial configuration
includes information that a local
domain name server (DNS)
can be
reached at grinnell.edu—IP number 132.161.10.11. If this server is
unavailable, it has the IP number for a secondary
network informa-
tion server (NIS)
. The DNS and NIS machines maintain databases
with domain names and IP numbers in a binary format that facili
tates quick lookup. In fact, for efficiency, this information is often
stored in two tables—one based on domain name and the other on
IP number—so that searches by either domain name or IP number
can be performed particularly fast.
In searching for the IP number for cs.utexas.edu, the local ma
chine first asks its local DNS, which is the authority for the local
network, but which may or may not know about the rest of the
world. In handling this request for information, this local DNS con
sults its tables to attempt to locate the needed IP number. If this in
formation is not in the table, the search fails, and the local machine
then asks its secondary NIS source, which is the authority for a
wider area of the network. If this fails as well, eventually a request
is generated for the DNS for all .edu addresses. This DNS then ei
ther returns the information requested or indicates that it is avail
able from a more specialized DNS, which handles information
down one branch of the domain name hierarchy. (For example, the
.edu DNS may indicate that information regarding cs.utexas.edu
can be obtained from a DNS specializing in utexas.edu, and give the
IP number for that corresponding specialized DNS.) In short, to de
termine the IP number that corresponds to a given domain name, a
