background image
OSI reference model
Open Systems Interconnection reference model: A
conceptual model defined by the International Organization for Standard-
ization (ISO), describing how any combination of devices can be connected
for the purpose of communication. The OSI model divides the task into
seven functional layers, forming a hierarchy with the applications at the top
and the physical medium at the bottom, and it defines the functions each
layer must provide. See also: Application layer, Data Link layer, Network layer,
Physical layer, Presentation layer, Session layer,
and Transport layer.
Open Shortest Path First: A link-state, hierarchical IGP routing algo-
rithm derived from an earlier version of the IS-IS protocol, whose features
include multipath routing, load balancing, and least-cost routing. OSPF is
the suggested successor to RIP in the Internet environment. See also:
Enhanced IGRP, IGP,
and IP.
OSPF areas
Small areas within an autonomous system that share routing
Organizationally Unique Identifier: Assigned by the IEEE to an orga-
nization that makes network interface cards. The organization then puts this
OUI on each and every card they manufacture. The OUI is 3 bytes (24 bits)
long. The manufacturer then adds a 3-byte identifier to uniquely identify the
host on an internetwork. The total length of the address is 48 bits (6 bytes)
and is called a hardware address or MAC address.
out-of-band management
Management "outside" of the network's
physical channels. For example, using a console connection not directly
interfaced through the local LAN or WAN or a dial-in modem. Compare to:
in-band management.
out-of-band signaling
Within a network, any transmission that uses
physical channels or frequencies separate from those ordinarily used for data
transfer. For example, the initial configuration of a Cisco Catalyst switch
requires an out-of-band connection via a console port.
In data communications, the basic logical unit of information
transferred. A packet consists of a certain number of data bytes, wrapped or
encapsulated in headers and/or trailers that contain information about
where the packet came from, where it's going, and so on. The various pro-
tocols involved in sending a transmission add their own layers of header
information, which the corresponding protocols in receiving devices then
Copyright ©2001 SYBEX , Inc., Alameda, CA