Information Technology Reference
In-Depth Information
local machine may consult several specialized DNS machines that
look up the desired information name in appropriate tables before
finding one that returns the appropriate IP number.
Of course, it might be that all designated DNS machines are un
available when your local computer attempts to find the IP number
that corresponds to a particular domain name. Because of this pos
sibility, some systems also maintain a table for some local machines
that are commonly accessed. Although this table is often in text for
mat rather than binary format, it can serve as a backup resource.
Local machines can refer to this system's table for IP numbers and
also can store IP numbers in this local table for future reference.
Once the local machine learns the IP number for the destination
of a message, it then must send the message itself. However, as
noted earlier, the IP number designates a logical machine, not the
physical connection of an Ethernet controller to a cable. Thus, an
other translation process is needed, and another table is used: The
machine needs to determine the Ethernet address associated with an
IP number. To identify the correct Ethernet address, the computer
consults another table—this one often stored in main memory.
At the start, this table is empty. Thus, the machine broadcasts a
message to all machines on its local network, asking “Which of you
has the following IP number: . . . ?” If the message is being sent to an
other machine in the local network, then the desired machine replies,
and the machine can send the message. If the machine is outside the
local network, no response is returned. Instead, the machine packages
the message in a packet to a default router, which in turn can follow
a similar process until the packet is delivered to the correct location.
In summary, Internet communication requires at least two types
of lookups: one to receive IP numbers from domain names, and one
to obtain Ethernet addresses from IP numbers. Because the Internet
involves a vast number of machines, it is impractical to keep all of this
information in a small local computer. Instead, a local machine may
keep a table of recently obtained information, because it typically
needs only a few Ethernet addresses at a time. These addresses are of
ten kept in a small table in main memory, where a simple search can
find the needed information. The computer will then consult other
machines when data are unknown. These central authorities for do
main names are (as we discussed) domain name servers, which use
specialized tables for fast retrieval of information.
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