T HE N ETWORK P RESS
ENCYCLOPEDIA
OF NETWORKING
S ECOND E DITION
W ERNER F EIBEL
N OW I MPROVED — THE M OST C OMPREHENSIVE
C OMPENDIUM OF N ETWORKING C ONCEPTS ,
I SSUES ,
AND T ERMS
C OVERS N ET W ARE 4.1, W INDOWS 95,
W INDOWS NT S ERVER 3.51, U NIX W ARE 2,
AND OS/2 W ARP C ONNECT
F ULL T EXT ON CD-ROM FOR Q UICK
E LECTRONIC R EFERENCE
The Encyclopedia
of Networking
The Encyclopedia
of Networking
Second Edition
The First Edition of this
book was published under
the title Novell’s® Complete
Encyclopedia of Networking
Werner Feibel
San Francisco s Paris s Düsseldorf s Soest
Acquisitions Editor: Kristine Plachy
Developmental Editor: Guy Hart-Davis
Editors: Kristen Vanberg-Wolff and Maureen Adams
Technical Editor: Mary Madden
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[In] a certain Chinese encyclopedia…it is written that animals are divided into:
(a) those belonging to the Emperor
(b) those that are embalmed
(c) tame ones
(d) suckling pigs
(e) sirens
(f) fabulous ones
(g) stray dogs
(h) those included in the present classification
(i) those that tremble as if mad
(j) innumerable ones
(k) those drawn with a very fine camelhair brush
(l) others
(m) those that have just broken the water pitcher
(n) those that look like flies from a long way off
Jorge Luis Borges
Acknowledgments
As with the first edition, this book would never have been completed without the help of
many people. These people deserve thanks for all their efforts and energy. Guy Hart-Davis
convinced me that it was time for a revision and set me to work. Several people did splen-
did work during the production process: Kris Vanberg-Wolff, a veteran of the first edition,
worked on the revision until her planned departure for the calmer (and tastier) world of
cooking school. Maureen Adams, Laura Arendal, and Nathan Johanson took over the pro-
duction chores after Kris left. They did an excellent job, especially considering the short
notice and even shorter revision schedule. My heartfelt thanks to all these folks.
Mary Madden’s technical reviews were always full of gentle, constructive corrections
and useful suggestions for improvements. Although I may not have been smart enough to
act on all of them, the suggestions have improved the book immensely—for which I’m very
grateful.
Kris Vanberg-Wolff’s eagle eyes and infallible grammatical sense found and fixed my
awkward phrasings, stylistic inconsistencies, and grammatical aberrations. I shudder to
think what the book would have looked like without the benefit of these efforts.
As always, I’m very grateful to all the people who worked between and behind the
scenes to make this book, and also to those who created the compact disc. Thanks also to
the many people who sent me information about their products and who took the time
to answer my questions.
Finally, I dedicate this book to my wife Luanne and my daughter Molly—for all the joy
and fun they provide, during both work and play hours.
Table of Contents
Introduction ix
Entries (Listed Alphabetically) 1
Appendix A: Acronyms and Abbreviations 1113
Appendix B: Bibliography and Other Resources 1235
Index 1251
Introduction
Introduction
Introduction
w
What You’ll Find in This Book
As in the first edition, I’ve tried to make this Encyclopedia a comprehensive source of informa-
tion about matters relating to networking. I’ve also tried to present the information in a clear
and useful manner.
This book contains comprehensive, straightforward summaries of the major concepts,
issues, and approaches related to networking. Networking is defined broadly to encompass
configurations ranging from a couple of connected computers just a few feet apart to a network
of several thousand machines (of all types and sizes) scattered around the world. You’ll find
discussions of networking as it’s done by servers and clients, managers and agents, peers, and
even over the telephone.
You probably won’t find anything here that you can’t find in other places. However, I don’t
know of any other book or source that collects so much network-related information in one
place. To find all the information summarized here, you would need to check hundreds of
books, disks, articles, Web pages, or other documents.
Despite its hefty size, this encyclopedia just scratches the surface of what there is to know
about networking. After all, how complete can any book be if just the World Wide Web on the
Internet has over 10 million hypertext documents. I do think, however, that this book scratches
deeper than most other references you’ll find.
This revised edition updates entries for concepts and technologies that change rapidly or
where there have been major developments. I’ve also added considerable material about the
Internet (and especially about the World Wide Web), since interest in this networking phenom-
enon is growing at an astounding pace.
Concepts, Not Instructions
As in the first edition, I’ve tried to cover concepts rather than making this a how-to book.
Thus, you won’t learn how to install networks or run specific programs. However, you will
learn about different types of programs and what they do. For example, you can read about
browsers and how they make exploring the World Wide Web possible; you’ll also learn about
programs such as network operating systems and how they differ from ordinary operating
systems.
x Introduction
An Anchor in an Ocean of Words
This book was obsolete from the moment it was written. That’s because nothing changes faster
than vocabulary in a field where there is money to be made. Since major breakthroughs and
advances are still happening in the area of networking, there are new network-related words
and concepts to be found in almost every issue of every computer magazine. If you include
acronyms and abbreviations, the speed with which the vernacular expands is even faster. For
example, the first edition of this book was published under a year ago and it was no trouble
finding almost 2,000 new entries for Appendix A.
Given the futility of even trying to stay completely up-to-date, I’ve chosen to focus on the
more enduring concepts and facts—those that provide the foundations and background that
underlie the constantly changing terminology. This makes the Encyclopedia more generally
useful and enduring.
Helping the Book Grow
While core networking concepts change very little, the core does grow. For example, ten years
ago there was much less need to know about wireless communications because there were
fewer wireless products, as well as less public interest in the technology. Because of such
progress, the body of essential fundamentals grows with each year.
I expect to update and add to the material in the book, and hope to make the Encyclopedia
always effective, comprehensive, and useful. Fortunately, an electronic medium makes it easier
to grow in this way.
If you need to find out something about networking, look for it in this book. If you find an
entry for the topic, we hope you’ll be more informed after you’ve read it. On the other hand,
if you can’t find the information you need, didn’t understand it, or don’t think you learned
what you should have, please drop us a line and tell us.
Also, if there are concepts or terms you would like to see included, please let us know. If you
can provide references, that would be helpful. Even under the best of circumstances, there’s lit-
tle chance that you’ll get a reply to individual queries. However, we will read your comments
and suggestions and will try to use them to improve future versions of the book.
Symbols &
Numbers
&
2 & (Ampersand)
Symbols & Numbers
w w
& (Ampersand) @ (At sign)
The ampersand is used to indicate special The at sign is used to separate the username
characters in HTML (Hypertext Markup from domain specifiers in e-mail addresses.
Language) documents—that is, documents For example,
[email protected]
for the World Wide Web. For example, would indicate someone with username mels
& specifies the ampersand character on a computer named golemxiv at MIT.
(&); ö specifies a lowercase o with w
an umlaut, or dieresis, mark (ö). \ (Backslash)
w In some operating systems, such as DOS,
< > (Angle Brackets) OS/2, and NetWare, the backslash character
Angle brackets are used in pairs to surround separates directory names or directory and
markup tags in HTML (Hypertext Markup file names in a path statement. By itself, the
Language) documents for the World Wide backslash represents the root directory in
Web. For example, indicates a para- these operating systems.
graph break; and indicate the start In various programming and editing con-
and end of a section that is to be displayed texts, the backslash is used to escape the
in boldface. character that follows. For example, \n is an
escape code to indicate a newline character
w in many operating environments.
* (Asterisk)
w
In several operating systems, the asterisk
// (Double Slash)
serves as a wildcard character: to represent
one or more characters, such as in a file In URLs (Uniform Resource Locators), dou-
name or extension. For example, a* matches ble slash characters separate the protocol
act, actor, and and, but not band. from the site and document names. For
In pattern matching involving regular example, if it existed,
expressions, the asterisk matches the occur- http://examplehost.ucsc.edu/
rences of the single character immediately filename.html
preceding it. For example, ba*th matches
bth, bath, and baaaaath, but not bbath. would refer to a file named filename.html
In e-mail and in other contexts that use residing on the examplehost machine at the
plain text, asterisks are sometimes used University of California at Santa Cruz. To
around words or phrases to indicate em- get to this file, you would use a server that
phasis. For example, “I *really* want supports the HTTP (Hypertext Transport
to emphasize the second word in this Protocol).
sentence.”
4B/5B Encoding 3
w events that have helped define the computer
µ(Mu) culture:
Used as an abbreviation for the prefix micro, http://www.phil.uni-sb.de/fun/jargon/
as in µsec for microsecond and µm for index.html
micrometer. This order of magnitude corre- In this URL, the file is named index.html,
sponds to 2−20, which is roughly 10−6, or and it is located in the /fun/jargon directory
one-millionth. on a machine in Germany (de).
SEE ALSO In other operating systems, such as DOS,
Order of Magnitude OS/2, and NetWare, a slash is sometimes
used to indicate or separate command line
w switches or options for a command.
. and .. (Period and Double Period)
w
In hierarchically organized directory sys-
1Base5
tems, such as those used by UNIX, DOS,
and OS/2, . and .. refer to the current and The IEEE 802.3 committee’s designation
the parent directories, respectively. In pat- for an Ethernet network that operates at
tern matching involving regular expressions, 1 megabit per second (Mbps) and that
the . matches any single character, except a uses unshielded twisted-pair (UTP) cable.
newline character. This configuration uses a physical bus,
with nodes attached to a common cable.
w AT&T’s StarLAN is an example of a 1Base5
? (Question Mark) network.
In many operating systems, a question mark
SEE ALSO
serves as a wildcard character that repre-
10BaseX; 10Broad36
sents a single character, such as in a file or
directory name. w
4B/5B Encoding
w
/ (Slash) 4B/5B encoding is a data-translation scheme
that serves as a preliminary to signal encod-
The slash (also known as a forward slash or
ing in FDDI (Fiber Distributed Data Inter-
a virgule) separates directory levels in some
face) networks. In 4B/5B, every group of
operating systems (most notably UNIX), in
four bits is represented as a five-bit symbol.
addresses for gopher, and in URLs (Uniform
This symbol is associated with a bit pattern
Resource Locators). For example, the fol-
that is then encoded using a standard signal-
lowing URL specifies the name and location
encoding method, usually NRZI (non-return
of a hypertext version of the jargon file,
to zero inverted).
which contains definitions for terms and
This preprocessing makes the subsequent
electrical encoding 80 percent efficient. For
4 5B/6B Encoding
example, using 4B/5B encoding, you can w
achieve a 100 megabit per second (Mbps) 10BaseX
transmission rate with a clock speed of only The designations 10Base2, 10Base5,
125 megahertz (MHz). 10BaseF, and 10BaseT refer to various
In contrast, the Manchester signal- types of baseband Ethernet networks.
encoding method, which is used in Ethernet
and other types of networks, is only 50 per- 10Base2
cent efficient. For example, to achieve a 100
10Base2 uses thin coaxial cable. This ver-
Mbps rate with Manchester encoding, you
sion can operate at up to 10 megabits per
need a 200 MHz clock speed.
second (Mbps) and can support cable seg-
w ments of up to 185 meters (607 feet). It is
5B/6B Encoding also known as thin Ethernet, ThinNet, or
A data-translation scheme that serves CheaperNet, because thin coaxial cable is
as a preliminary to signal encoding in considerably less expensive than the thick
100BaseVG networks. In 5B/6B, every coaxial cable used in 10Base5 networks.
group of five bits is represented as a six-bit 10Base5
symbol. This symbol is associated with a bit
pattern that is then encoded using a stan- 10Base5 uses thick coaxial cable. This ver-
dard signal-encoding method, such as NRZ sion is the original Ethernet. It can operate
(non-return to zero). at up to 10 Mbps and support cable seg-
ments of up to 500 meters (1,640 feet). It is
w
8B/10B Encoding also known as thick Ethernet or ThickNet.
A data-translation scheme related to 4B/5B 10BaseF
encoding that recodes eight-bit patterns into
10BaseF is a baseband 802.3-based Ethernet
10-bit symbols. 8B/10B encoding is used, for
network that uses fiber-optic cable. This
example, in IBM’s SNA (Systems Network
version can operate at up to 10 Mbps.
Architecture) networks.
Standards for the following special-
w purpose versions of 10BaseF are being
9-Track Tape formulated by the IEEE 802.3:
A tape storage format that uses nine parallel 10BaseFP (fiber passive): For desktops
tracks on 1/2-inch, reel-to-reel magnetic
tape. Eight tracks are used for data, and one 10BaseFL (fiber link): For intermediate
track is used for parity information. These hubs and workgroups
tapes are often used as backup systems on 10BaseFB (fiber backbone): For central
minicomputer and mainframe systems; digi- facility lines between buildings
tal audio tapes (DATs) are more common on
networks.
66-Type Punch-Down Block 5
10BaseT cables for each direction, so that each cable
needs only an 18 MHz bandwidth.
10BaseT is a baseband 802.3-based Ethernet
network that uses unshielded twisted-pair B RO A D E R C A T E G O R I E S
(UTP) cable and a star topology. This ver- Ethernet; Network, Broadband
sion can operate at up to 10 Mbps. It is also SEE ALSO
known as twisted-pair Ethernet or UTP 1Base5; 10BaseX
Ethernet.
w
B RO A D E R C A T E G O R Y 56K Line
Ethernet
A digital telephone circuit with a 64 Kbps
SEE ALSO bandwidth, but with a bandwidth of only
1Base5; 10Broad36; 100BaseT 56 Kbps data, with the other 8 Kbps being
used for signaling. Also known as an ADN
w
(Advanced Digital Network) or a DDS
10Broad36
(Dataphone Digital Service) line.
10Broad36 is a broadband, 802.3-based,
Ethernet network that uses 75-ohm coaxial w
(CATV) cable and a bus or tree topology.
64K Line
This version can operate at up to 10 mega- A digital telephone circuit with a 64 Kbps
bits per second (Mbps) and support cable bandwidth. Also known as a DS0 (digital
segments of up to 1,800 meters (about signal, level 0) line. When the entire 64 Kbps
6,000 feet). are allocated for the data, the circuit is
A 10Broad36 network uses differential known as a clear channel. This is in contrast
phase shift keying (DPSK) to convert the to a circuit in which 8 Kbps are used for
data to analog form for transmission. signaling, leaving only 56 Kbps for data.
Because of the encoding details, a
w
10Broad36 network actually needs 66-Type Punch-Down Block
18 megahertz (MHz) for each channel:
14 MHz to encode the 10 Mbps signal and A device for terminating wires, with the
4 MHz more for collision detection and possibility of connecting input and output
reporting capabilities. wires. This type of punch-down block can
In a 10Broad36 network, throughput is handle wires with up to 25 twisted pairs.
10 Mbps in each direction—that is, a total The 66-type have generally been superseded
bandwidth of 36 MHz is needed. This band- by 110-type punch-down blocks.
width can be provided in a single cable or in SEE ALSO
two separate cables. A split-cable approach Punch-Down Block
uses half the cable for each direction, which
means the cable must have a 36 MHz band-
width. A dual-cable approach uses separate
6 100BaseFX
w The main differences between fast (100
100BaseFX Mbps) Ethernet and standard (10 Mbps)
A 100BaseT basal type variant that runs Ethernet are:
over multimode fiber-optic cable. Nodes on s A 100BaseT Ethernet allows a much
a 100BaseFX network can be up to 2 kilo- shorter gap between signals.
meters apart. This variant is also written
100Base-FX. s A 100BaseT Ethernet requires either
higher-grade cable or more wire pairs.
SEE
It can run at 100 Mbps speeds on
100BaseT Category 3 or 4 cable—provided four
C O M P A RE pairs are available; Category 5 cable
100BaseT4; 100BaseTX requires only two pairs.
s Currently, a 100BaseT Ethernet can
w
100BaseT support a network that is only about
a tenth of the length allowed for an
The general name for any of three 100 Mbps
ordinary Ethernet network. For net-
Ethernet variants that have just been made a
works that use copper (as opposed to
standard by an IEEE 802.3 subcommittee
fiber-optic) cabling: Two nodes of a
(802.3u). 100BaseT Ethernet is one of the
100BaseT4 network can be no further
candidates trying to become the standard
apart than 205 meters—regardless of
100 Mbps Ethernet. This version was devel-
whether the nodes are next to each
oped and proposed originally by Grand
other.
Junction, in collaboration with several other
corporations. The following variants of 100BaseT
The term fast Ethernet is often used for Ethernet have been defined:
this version. This is unfortunate, since that
100BaseFX: Runs over multimode fiber-
term is also used to refer to any Ethernet
optic cable. Nodes on a 100BaseFX
implementation that supports speeds faster
network can be up to two kilometers
than the official 10 Mbps standard. To add
apart.
to the confusing terminology, a software
product (no longer available) was also 100BaseTX: Uses two wire pairs,
named fastEthernet. but requires Category 5 unshielded
100BaseT Ethernet retains Ethernet’s or shielded twisted pair (UTP or
CSMA/CD (Carrier Sense Multiple Access/ STP) wire.
Collision Detect) media access method—in
100BaseT4: Can use category 3, 4, or 5
contrast to the 100BaseVG variant (now
UTP cable. The T4 in the name comes
officially, IEEE 802.12)—which is the other
from the fact that four wire pairs are
major 100 Mbps Ethernet available.
needed: two for sending and two for
receiving.
100BaseVG 7
In some configurations, fast and ordinary SEE
Ethernet nodes can share the same network. 100BaseT
Fast Ethernet devices identify themselves as
C O M P A RE
such by sending a series of FLPs (fast link
100BaseT4; 100BaseFX
pulses) at startup.
w
P R I M A R Y S O U RC E S
100BaseVG
IEEE 802.3u committee publications
100BaseVG is a version of Ethernet devel-
B RO A D E R C A T E G O R I E S oped by Hewlett-Packard (HP) and AT&T
Ethernet Microelectronics, and is currently under
C O M P A RE
consideration by an IEEE 802.12 committee.
It is an extension of 10BaseT Ethernet that
100BaseVG
will support transmissions of up to 100
w megabits per second (Mbps) over voice-
100BaseT4 grade (Category 3) twisted-pair wire. The
A 100BaseT Ethernet variant that can use VG in the name stands for voice-grade.
category 3, 4, or 5 unshielded twisted pair
Differences from 10 Mbps Ethernet
(UTP) cable. The T4 means that four wire
pairs are needed: two for sending and two 100BaseVG Ethernet differs from ordinary
for receiving. Two nodes of a 100BaseT4 (10 Mbps) Ethernet in the following ways:
network can be no further apart than 205
s Uses demand priority (rather than
meters, regardless of whether the nodes are
CSMA/CD) as the media access
next to each other. This variant is sometimes
method.
written 100Base-T4.
s Can use ordinary (Category 3)
SEE
unshielded twisted-pair (UTP) cable,
100BaseT
provided that the cable has at least
C O M P A RE four wire pairs. Ordinary Ethernet
100BaseTX; 100BaseFX needs only two pairs: one to send and
one to receive.
w
100BaseTX s Uses quartet signaling to provide four
A 100BaseT Ethernet variant that uses two transmission channels (wire pairs)
wire pairs, but requires Category 5 UTP or instead of just one. All wire pairs are
STP wire. Two nodes of a 100BaseTX net- used in the same direction at a given
work can be no further apart than 205 time.
meters—regardless of whether the nodes are s Uses the more efficient 5B/6B NRZ
next to each other. This variant is sometimes signal encoding, as opposed to the
written 100Base-TX.
8 100BaseX
Manchester encoding scheme used by 100BaseVG/AnyLAN
ordinary Ethernet.
100BaseVG/AnyLAN is an extension
s For category 3 cable, a VG network of 100BaseVG, developed as a joint effort
can be at most 600 meters from end to between Hewlett-Packard and IBM. This
end—and only 200 meters if all hubs version also supports the Token Ring archi-
in the network are connected in the tecture, and it can be used with either Ether-
same wiring closet. These values net or Token Ring cards (but not both at the
increase by 50%—that is, to 900 and same time or in the same network). Because
300 meters, respectively—when cate- the demand priority access method can be
gory 5 cable is used. For VG using deterministic, the 100BaseVG/AnyLAN
fiber-optic cable, the most widely sepa- architecture could handle isochronous
rated network nodes can be up to data—that is, data (such as voice or video)
5000 meters, or 5 kilometers, apart. that requires a constant transmission rate.
Upgrading to 100BaseVG The 100VG-AnyLAN Forum is the advo-
cacy group for this Ethernet variant. This
100BaseVG is designed to provide an easy consortium includes over 20 members,
upgrade path from 10 Mbps Ethernet. An including Apple, Compaq, and IBM.
upgrade requires two new components: 100Base VG/AnyLAN is also known simply
as VG or AnyLAN.
s A 100BaseVG network interface card
(NIC) for each node being upgraded. B RO A D E R C A T E G O R Y
This NIC replaces the 10 Mbps version Ethernet
in the node.
SEE ALSO
s A 100BaseVG hub to replace the 10 HSLAN (High-Speed Local-Area
Mbps hub. This type of hub is plug- Network)
compatible with a 10 Mbps hub, so
C O M P A RE
that the upgrade requires simply
unplugging a node from one hub and 100BaseT
plugging it into the 100BaseVG hub. w
This can all take place in the wiring 100BaseX
closet.
100BaseX (sometimes written as 100
If you are already using twisted-pair Base-X) is a function that translates bet-
Ethernet cabling, you may not need any ween the FDDI (Fiber Distributed Data
new wiring, provided that the cable has four Interface)-based physical layer and the
wire pairs. CSMA/CD-based data-link layer in a 100
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