13
address classes
to changing line conditions by changing its
data rate. As line quality improves, the mo-
dem attempts to increase the data rate; as
line quality declines, the modem compen-
sates by lowering the data rate. Also known
as adaptive equalization.
ADCCP
See Advanced Data Communi-
cations Control Procedures.
address 1. The precise location in mem-
ory or on disk where a piece of information
is stored. Each byte in memory and each
sector on a disk has its own unique address.
2. The unique identifier for a specific node
on a network. An address may be a physical
address specified by switches or jumpers on
the network interface card hardware, or it
can be a logical address established by the
network operating system.
3. To reference or manage a storage
location.
4. In Unix, an IP address as specified in the
/etc/hosts
file.
5. Information used by a network or the In-
ternet to specify a specific location in the
form
username@hostname; username is
your user name, logon name, or account
name or number, and
hostname is the name
of the Internet Service Provider (ISP) or
computer system you use. The hostname
may consist of several parts, each separated
from the next by a period.
See also address bus; Domain Name Ser-
vice; e-mail address; Internet Service Pro-
vider; IP address; memory address.
address bus The electronic channel,
usually from 20 to 64 lines wide, used to
transmit the signals that specify locations in
memory.
The number of lines in the address bus de-
termines the number of memory locations
that the processor can access, because each
line carries one bit of the address. A 20-line
address bus (used in early Intel 8086/8088
processors) can access 1MB of memory, a
24-line address bus can access 16MB, and a
32-line address bus can access more than
4GB. A 64-line address bus (used in the
DEC Alpha APX) can access 16EB.
address classes In a 32-bit IP address,
which is shown in the accompanying illus-
tration, the number of bits used to identify
the network and the host vary according
to the network class of the address, as
follows:
■
Class A is used only for very large net-
works. The high-order bit in a Class A
network is always zero, leaving 7 bits
available to define 127 networks. The re-
maining 24 bits of the address allow each
Class A network to hold as many as
16,777,216 hosts. Examples of Class A
networks include General Electric, IBM,
Hewlett-Packard, Apple Computer,
Xerox, Digital Equipment Corporation,
and MIT. All the Class A networks are in
use, and no more are available.
■
Class B is used for medium-sized net-
works. The 2 high-order bits are always
10, and the remaining bits are used to de-
fine 16,384 networks, each with as many
as 65,535 hosts attached. Examples of
Class B networks include Microsoft and
Exxon. All Class B networks are in use,
and no more are available.
2461book Page 13 Thursday, May 4, 2000 11:59 AM
Copyright © 2000 SYBEX Inc., Alameda, CA.