Cabletron Systems Fast Network 10 User manual

FAST NETWORK 10
USER GUIDE
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Fast Network 10 User Guide i
NOTICE
Cabletron Systems reserves the right to make changes in specifications and other information
contained in this document without prior notice. The reader should in all cases consult Cabletron
Systems to determine whether any such changes have been made.
The hardware, firmware, or software described in this manual is subject to change without notice.
IN NO EVENT SHALL CABLETRON SYSTEMS BE LIABLE FOR ANY INCIDENTAL,
INDIRECT, SPECIAL, OR CONSEQUENTIAL DAMAGES WHATSOEVER (INCLUDING BUT
NOT LIMITED TO LOST PROFITS) ARISING OUT OF OR RELATED TO THIS MANUAL OR
THE INFORMATION CONTAINED IN IT, EVEN IF CABLETRON SYSTEMS HAS BEEN
ADVISED OF, KNOWN, OR SHOULD HAVE KNOWN, THE POSSIBILITY OF SUCH
DAMAGES.
 Copyright March 1996 by Cabletron Systems, Inc., P.O. Box 5005, Rochester, NH 03866-5005
All Rights Reserved
Printed in the United States of America
Order Number: 9031805-01 May 1996
All other product names mentioned in this manual may be trademarks or registered trademarks of
their respective companies.
Printed on Recycled Paper

ii Fast Network 10 User Guide
FCC NOTICE
This device complies with Part 15 of the FCC rules. Operation is subject to the following two
conditions: (1) this device may not cause harmful interference, and (2) this device must accept any
interference received, including interference that may cause undesired operation.
NOTE: This equipment has been tested and found to comply with the limits for a Class A digital
device, pursuant to Part 15 of the FCC rules. These limits are designed to provide reasonable
protection against harmful interference when the equipment is operated in a commercial environment.
This equipment uses, generates, and can radiate radio frequency energy and if not installed in
accordance with the operator’s manual, may cause harmful interference to radio communications.
Operation of this equipment in a residential area is likely to cause interference in which case the user
will be required to correct the interference at his own expense.
WARNING: Changes or modifications made to this device which are not expressly approved by the
party responsible for compliance could void the user’s authority to operate the equipment.
DOC NOTICE
This digital apparatus does not exceed the ClassA limits for radio noise emissions from digital
apparatus set out in the Radio Interference Regulations of the Canadian Department of
Communications.
Le présent appareil numérique n’émet pas de bruits radioélectriques dépassant les limites applicables
aux appareils numériques de la class A prescrites dans le Règlement sur le brouillage radioélectrique
édicté par le ministère des Communications du Canada.
VCCI NOTICE
This equipment is in the 1st Class Category (information equipment to be used in commercial and/or
industrial areas) and conforms to the standards set by the Voluntary Control Council for Interference
by Information Technology Equipment (VCCI) aimed at preventing radio interference in commercial
and/or industrial areas.
Consequently, when used in a residential area or in an adjacent area thereto, radio interference may be
caused to radios and TV receivers, etc.
Read the instructions for correct handling.

Fast Network 10 User Guide iii
CABLETRON SYSTEMS, INC. PROGRAM LICENSE AGREEMENT
IMPORTANT: Before utilizing this product, carefully read this License Agreement.
This document is an agreement between you, the end user, and Cabletron Systems, Inc. (“Cabletron”)
that sets forth your rights and obligations with respect to the Cabletron software program (the
“Program”) contained in this package. The Program may be contained in firmware, chips or other
media. BY UTILIZING THE ENCLOSED PRODUCT,YOUARE AGREEING TO BECOME
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AGREE TO THE TERMS OF THIS AGREEMENT, PROMPTLY RETURN THE UNUSED
PRODUCT TO THE PLACE OF PURCHASE FOR A FULL REFUND.
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You may not copy, reproduce or transmit any part of the Program except as permitted by the
Copyright Act of the United States or as authorized in writing by Cabletron.
2. OTHER RESTRICTIONS. You may not reverse engineer, decompile, or disassemble the
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3. APPLICABLE LAW. This License Agreement shall be interpreted and governed under the laws
and in the state and federal courts of New Hampshire. You accept the personal jurisdiction and
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EXCLUSION OF WARRANTY AND DISCLAIMER OF LIABILITY
1. EXCLUSION OF WARRANTY. Except as may be specifically provided by Cabletron in
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CABLETRON DISCLAIMS ALL WARRANTIES, OTHER THAN THOSE SUPPLIED TO
YOU BY CABLETRON IN WRITING, EITHER EXPRESSED OR IMPLIED, INCLUDING
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YOU.

iv Fast Network 10 User Guide
UNITED STATES GOVERNMENT RESTRICTED RIGHTS
The enclosed product (a) was developed solely at private expense; (b) contains “restricted computer
software” submitted with restricted rights in accordance with Section 52227-19 (a) through (d) of the
Commercial Computer Software - Restricted Rights Clause and its successors, and (c) in all respects
is proprietary data belonging to Cabletron and/or its suppliers.
For Department of Defense units, the product is licensed with “Restricted Rights” as defined in the
DoD Supplement to the Federal Acquisition Regulations, Section 52.227-7013 (c) (1) (ii) and its
successors, and use, duplication, disclosure by the Government is subject to restrictions as set forth in
subparagraph (c) (1) (ii) of the Rights in Technical Data and Computer Software clause at
252.227-7013. Cabletron Systems, Inc., 35 Industrial Way, Rochester, New Hampshire 03867-0505.

FastNET 10 User Guide v
CONTENTS
CHAPTER 1 INTRODUCTION
1.1 About This Manual.......................................................................1-1
1.2 Getting Help.................................................................................1-2
1.3 Document Conventions...............................................................1-3
1.4 Related Documentation...............................................................1-4
1.5 Overview......................................................................................1-4
1.5.1 FN10 Architecture...........................................................1-6
1.5.1.1 Store and Forward Switching.........................1-7
1.5.1.2 Discarding Local Traffic..................................1-8
1.5.1.3 Spanning Tree Algorithm................................1-8
1.5.2 FN10 Bridge Address Table ...........................................1-9
1.5.3 FN10 Filtering ...............................................................1-10
1.5.4 FN10 Sample Applications ...........................................1-11
1.5.4.1 FN10 Trunking..............................................1-11
1.5.4.2 FN10’s Fast Ethernet Option........................1-12
1.5.4.3 Virtual Workgroups.......................................1-14
1.6 Local Console Manager.............................................................1-15
1.6.1 Command Syntax Conventions ....................................1-16
1.6.2 Basic LCM Commands.................................................1-17
1.6.2.1 Help..............................................................1-18
1.6.2.2 Erase............................................................1-18
1.6.2.3 Exit ...............................................................1-19
1.6.2.4 Logout ..........................................................1-19
1.6.2.5 Traplog.........................................................1-19
CHAPTER 2 UNPACKING AND INSTALLINGYOUR FN10
2.1 FN10 Panels................................................................................2-1
2.2 Installing the FN10.......................................................................2-4
2.2.1 Checking the Power-up Diagnostics Sequence..............2-6
2.3 Connecting the Local Console Manager .....................................2-7
2.4 Connecting the FN10 to the Network ..........................................2-8
2.4.1 Connecting the AUI Interface..........................................2-9
2.5 Adding or Replacing the Optional Fast Ethernet Module ............2-9

Contents
vi FastNET 10 User Guide
CHAPTER 3 CONFIGURINGYOUR FN10
3.1 Assigning IP Addresses...............................................................3-3
3.1.1 Displaying IP Addresses .................................................3-4
3.1.2 Deleting an IP Address....................................................3-4
3.1.3 Changing a Subnet Mask................................................3-4
3.2 Enabling Bridging.........................................................................3-5
3.3 Disabling Bridging........................................................................3-6
3.4 Displaying Bridging Functions......................................................3-6
3.5 Enabling Trunking........................................................................3-7
3.6 Disabling Trunking .......................................................................3-9
3.7 Displaying Trunking Status ........................................................3-10
3.8 Defining and Deleting Workgroups ............................................3-12
3.9 Assigning a Community Name...................................................3-15
3.10 Configuring Multicast Storm Protection......................................3-16
3.11 Modifying MIB Variables ............................................................3-17
3.11.1 System Contact.............................................................3-17
3.11.2 System Name................................................................3-17
3.11.3 System Location............................................................3-17
3.11.4 Authentication Password...............................................3-18
3.11.5 Aging Parameter ...........................................................3-18
CHAPTER 4 MONITORING AND MANAGINGYOUR FN10
4.1 FN10 Management Tools ............................................................4-1
4.2 FN10 Statistics.............................................................................4-2
4.2.1 Pseudo Filters .................................................................4-3
4.2.2 Gathering Statistics.........................................................4-3
4.2.3 System Statistics.............................................................4-3
4.2.4 Ethernet Port Statistics....................................................4-4
4.2.5 MAC Statistics.................................................................4-6
4.2.6 Traffic Analysis Statistics.................................................4-7
4.2.7 SNMP Statistics...............................................................4-7
4.3 Using LCM to Check FN10 Status...............................................4-9
4.3.1 Displaying Status.............................................................4-9
4.3.2 Displaying MAC Addresses...........................................4-11
4.3.3 Displaying Manufacturing Information...........................4-14
4.4 Managing the FN10 ...................................................................4-14

Contents
FastNET 10 User Guide vii
4.5 Using LCM to Manage the FN10...............................................4-15
4.5.1 Disabling a Port ............................................................4-15
4.5.2 Enabling a Port .............................................................4-16
4.5.2.1 noRIP Option................................................4-16
4.5.3 Changing a Subnet Mask .............................................4-17
4.5.4 Changing a Community Name......................................4-18
4.5.5 Setting the Baud Rate...................................................4-18
4.5.6 Setting a Reboot Time..................................................4-19
CHAPTER 5 FN10 FILTERS
5.1 Bridge Address Table Filters.......................................................5-1
5.1.1 Source Address Filter .....................................................5-3
5.1.2 Source Address Multicast Filter......................................5-3
5.1.3 Destination Address Filter...............................................5-4
5.2 Port Filters...................................................................................5-4
5.2.1 Configurable Fields.........................................................5-5
5.2.1.1 Pseudo Filtering .............................................5-6
5.2.1.2 Filter Links......................................................5-6
5.3 Using Filters for Security Purposes...........................................5-10
5.4 Using Filters to Enhance Network Performance........................5-16
5.5 Configuring a Port Filter.............................................................5-19
5.5.1 Modifying a Port Filter...................................................5-22
5.5.2 Deleting a Port Filter.....................................................5-23
5.6 Filtering and Performance Considerations................................5-23
CHAPTER 6 FN10 DIAGNOSTICS ANDTROUBLESHOOTING
6.1 Power-up Diagnostics..................................................................6-1
6.1.1 Power-up LED Sequence ...............................................6-2
6.1.2 Specific Power-up Tests.................................................6-2
6.1.3 Software Checksum Comparison ...................................6-3
6.1.4 Power-up Diagnostics Results........................................6-3
6.2 Responses to Failures at Power-up ............................................6-3
6.3 Diagnostic Loopback Tests .........................................................6-3
6.3.1 Loopback Tests ..............................................................6-4
6.4 Status and Activity Indicators ......................................................6-4
6.5 Troubleshooting...........................................................................6-7
6.5.1 FN10 Does Not Power Up ..............................................6-7
6.5.2 Connectivity Problems....................................................6-7
6.5.3 FN10 Has Rebooted.......................................................6-8
6.5.4 FN10 Does Not Respond to NMS...................................6-8

Contents
viii FastNET 10 User Guide
APPENDIX A TECHNICAL SPECIFICATIONS
A.1 FN10 Specifications....................................................................A-1
A.2 Serial Cable Pin Assignments.....................................................A-3
A.3 10BASE-T Pin Assignments....................................................... A-3
A.4 Straight-through Wiring...............................................................A-4
A.5 Crossover Wiring ........................................................................A-5
A.6 5 - 4 - 3 Rule ............................................................................... A-5
APPENDIX B GLOSSARY
INDEX

Fast Network 10 User Guide Page 1-1
CHAPTER 1
INTRODUCTION
1.1 ABOUTTHIS MANUAL
This manual is for system administrators responsible for configuring,
monitoring, and maintaining the Fast Network 10 (FN10).You should
have a familiarity with networking concepts and principles. In addition, a
basic understanding of SNMP is helpful.
Some FN10 configurations can only be done using an SNMP-based
Network Management System (NMS). Therefore, how you configure and
manage the FN10 is dependent on the NMS you use. Where applicable,
this manual provides instructions for using the Local Console Manager
(LCM) to perform basic configuration. Where it is not possible to use
LCM, general instructions and guidelines applicable to most NMSs are
provided.
The contents of each chapter are described below.
• Chapter 1,
Introduction , outlines the contents of this manual and
provides an overview of the FN10’s switching functions and the Local
Console Manager (LCM).
• Chapter 2,
Unpacking and Installing Your FN10 , describes the
FN10 front and rear panels, how to install the FN10, how to connect
the Local Console Manager (LCM), and how to connect the FN10 to
the network.
• Chapter 3,
Configuring Your FN10 , provides instructions for
configuringthe FN10usingthe LocalConsole Manager(LCM).It also
provides some common Management Information Base (MIB)
variables you may want to change.
• Chapter 4,
Monitoring and Managing Your FN10 , describes how to
monitor FN10 status and statistics. It also describes how to manage the
FN10 Ethernet ports using the Local Console Manager (LCM).

Chapter 1: Introduction
Page 1-2 Fast Network 10 User Guide
• Chapter 5, FN10 Filters , describes FN10 filtering and provides
specific examples of how filters can be used. It also provides
instructions for adding, modifying, and deleting Port filters using the
Local Console Manager (LCM).
• Chapter 6,
FN10 Diagnostics and Troubleshooting , describes the
FN10 diagnostics and provides information on troubleshooting
common problems.
• Appendix A,
Technical Specifications , provides the FN10
specifications and basic 10BASE-T cabling pin assignments.
• Appendix B, Glossary , provides a glossary of terms both specific to
the FN10 and common to the networking field.
1.2 GETTING HELP
If you need additional support related to the FN10, or if you have any
questions, comments, or suggestions concerning this manual, contact
Cabletron Systems Technical Support:
By phone [phone]
Monday-Friday; 8
A . M . – 8 P . M . Eastern Time
By CompuServe GO CTRON from any ! prompt
By Internet mail [email protected]
By FTP ctron.com (134.141.197.25)
Login
anonymous
Password your email address

Document Conventions
Fast Network 10 User Guide Page 1-3
1.3 DOCUMENT CONVENTIONS
The following conventions are used throughout this document:
LCM commands, prompts, and information displayed by the computer
appear in Courier typeface, for example:
Current Number of Learned Addresses: 133
Number of Defined Filters: 4
Information that you enter appears in Courier bold typeface, for example:
FN10 > status
Information that you need to enter with a command is enclosed in angle
brackets < >. For example, you must enter a port number and an IP
address to execute the
ipaddr <port #> <IP address>
command:
FN10 > ipaddr 6 192.138.217.40
Field value options appear in bold typeface. For example, a FN10 filter
type can be either
Entry or Exit .
NOTE
Note symbol. Calls the reader’s attention to any item of
information that may be of special importance.
TIP
Tip symbol. Conveys helpful hints concerning procedures or
actions.
!
CAUTION
Caution symbol. Contains information essential to avoid
damage to the equipment.
Warning symbol.Warns against an action that could result in
equipment damage, personal injury or death.

Chapter 1: Introduction
Page 1-4 Fast Network 10 User Guide
1.4 RELATED DOCUMENTATION
The following documentation may assist the user in using this product:
•
Fast Network 10 MIB Reference Guide – contains enterprise MIB
information.
• Interconnections, Bridges and Routers, Radia Perlman, Addison
Wesley © 1992.
• Internetworking with TCP/IP: Principles, Protocols, and Architecture
(2nd edition),Volumes I and II, Douglas Comer, Prentice Hall © 1991.
• The Simple Book, An Introduction to Management of TCP/IP-based
internets
, Marshall T. Rose, Prentice Hall © Second Edition, 1994.
1.5 OVERVIEW
The FN10 is an intelligent Ethernet-to-Ethernet switch that is available in
the following configuration options:
• 12 port - 12 IEEE 802.3 10BASE-T Ethernet ports, including one
Ethernet Attachment Unit Interface (AUI) connection.
• 12 port with FE up-link option - 2 Fast Ethernet ports (100 Mbps)
and 12 IEEE 802.3 10BASE-T Ethernet ports (10 Mbps), including
one Ethernet Attachment Unit Interface (AUI) connection.
•
24 port - 24 IEEE 802.3 10BASE-T Ethernet ports, including one
Ethernet Attachment Unit Interface (AUI) connection.
• 24 port with FE up-link option - 2 Fast Ethernet ports (100 Mbps),
and 24 IEEE 802.3 10BASE-T Ethernet ports (10 Mbps), including
one Ethernet Attachment Unit Interface (AUI) connection.
In addition, each FN10 configuration includes an RS232C port for
out-of-band management.
The following figures show the different front panels for the 12 and 24
port FN10 configurations, and the rear panel for the optional 2 Fast
Ethernet ports.

Overview
Fast Network 10 User Guide Page 1-5
.
Figure 1-1 FN10 Front Panels
Figure 1-2 FN10 Rear Panel with the Optional Fast Ethernet Ports
The FN10:
• Provides dedicated bandwidth for each network connected to its ports.
• Provides full store and forward bridging functionality.
• Provides complete error checking functionality.
• Provides port trunking to increase bandwidth.
• Allows you to define virtual workgroups to optimize network traffic.
• Filters and forwards received Ethernet packets based on Network
Management System (NMS) configurable parameters.
• Supports 48-bit IEEE 802 MAC addressing.
Front Panel with 12 10BASE-T (10 Mbps) Ports
Front Panel with 24 10BASE-T (10 Mbps) Ports
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Rear Panel with 2 Fast Ethernet (100 Mbps) Ports
Port B Port A
Link
Status
Ready
Port BPort A
Link
Link
Status Status Ready
BA
RX TX RX TX
RJ45
ST fiber-optic
Connection type options

Chapter 1: Introduction
Page 1-6 Fast Network 10 User Guide
• Implements the Spanning Tree protocol (802.1d).
• Configured with factory-set defaults for immediate plug-and-play
capability.
In addition, the FN10 offers features that can help you manage and
maintain your network, such as:
• Configuration and management using the Simple Network
Management Protocol (SNMP) with either an in-band or out-of-band
connection.
• Protection against multicast storms.
• Data flow control based on user-defined data packet filters.
• Ability to define virtual workgroups for more efficient bandwidth
usage.
• Compilation of statistics for traffic generated by each user device
connected to a FN10 segment.
• Real time “what-if” analysis of the traffic flow throughout the network.
1.5.1 FN10 Architecture
The FN10 enables you to link two or more Local Area Networks (LANs)
together. To accomplish this, the FN10 regulates network traffic on the
basis of the source and destination addresses that are in each data packet it
receives.
The FN10 is protocol-transparent, meaning it can handle different types
of network traffic regardless of the network protocol, such as IP and IPX.
As the FN10 reads addresses from the packets it processes, it builds a
dynamic database of addresses called the
Bridge Address Table . In this
way, the FN10 continuously learns the addresses of all connected devices.
Consequently, you can add new devices to the network, change device
addresses, and remove devices from the network without having to
reconfigure the FN10.
The Open System Interconnection (OSI) Reference Model, developed by
the International Standards Organization (ISO), identifies the levels of
functionality inherent in each of its seven layers.The FN10 operates at the

Overview
Fast Network 10 User Guide Page 1-7
Media Access Control (MAC) sub-layer of the Data Link layer.
Figure 1-3 shows the OSI Reference Model.
Figure 1-3 OSI Reference Model
Because the FN10 does not process any Network Layer information, it
provides a high level of performance in terms of packet throughput. In
addition, the FN10 does not need to learn network topology, requiring less
programming and configuration time.
1.5.1.1 Store and Forward Switching
As an intelligent Ethernet switch, the FN10 uses full store and forward
switching. Store and forward switching allows the FN10 to temporarily
store packets until network resources, typically an unused link, are
available for forwarding. This allows for complete error checking, and
limits the amount of time between when a device requests access to the
network and when it is granted permission to transmit. In addition, full
store and forward switching ensures data integrity, thus preventing
network error conditions from being generated throughout the network.
1.5.1.2 Discarding LocalTraffic
The FN10 checks all incoming packets for their destination address
against the Bridge Address Table. If a packet’s destination address is not
on the same network segment as the originating packet, the FN10
6Presentation
7
5
4
3
2
1
Application
Session
Transport
Network
Data Link
Physical
FN10 operates at Layer 2

Chapter 1: Introduction
Page 1-8 Fast Network 10 User Guide
forwards the packet to the network segment associated with that
destination address. However, if the packet’s source and destination
address are on the same network segment, known as
local traffic , the
packet is automatically discarded (i.e., ignored by the FN10).
For example, a file transmitted from Workstation A to Workstation C in
Figure 1-4 does not need to leave LAN 1. The FN10 connected to LANs 1
and 2 sees all traffic from LAN 1, including LAN 1 local traffic.
Figure 1-4 Typical Switching Application
By forwarding only packets addressed to devices on other network
segments, the FN10 reduces unnecessary traffic and thereby enhances the
overall performance of the network.
1.5.1.3 SpanningTree Algorithm
The FN10 supports the IEEE 802.1d Spanning Tree algorithm. The
Spanning Tree algorithm converts multiple LANs into a “spanning tree”
of networks. It is used to prevent bridging loops. This standard defines a
logical (not physical) network configuration consisting of one extended
LAN without active duplicate paths between spanning tree bridges.
The FN10, along with other IEEE 802.1d Spanning Tree compliant
bridges in the network, dynamically configure the network topology into
a single Spanning Tree by exchanging Bridge Protocol Data Units
(BPDUs). Typically, each LAN segment is sent one BPDU every two
seconds.
When there are multiple FN10s connecting LANs in a loop, the Spanning
Tree algorithm determines which FN10 should forward packets to the
LAN. If there is a cable break or a port failure, the network topology is
LAN 1 LAN 2
ABC
FastNet 10
FN10

Overview
Fast Network 10 User Guide Page 1-9
automatically reconfigured by the Spanning Tree protocol to create an
alternate path to the LAN.
1.5.2 FN10 Bridge AddressTable
The FN10 creates and maintains a dynamic database of addresses called
the Bridge Address Table. The FN10 examines every packet to determine
its source address and LAN segment origin. It then compares the source
address and segment information it finds to the entries in the Bridge
Address Table.
If a packet’s address is not already stored in the BridgeAddress Table, the
FN10 adds the learned address, associated segment number, and a timer
value that indicates the age of the observation. Consequently, the FN10
knows the address and associated segment number the next time it sees
that address. By using the information stored in the BridgeAddress Table,
the FN10 is able to quickly forward each packet to the correct LAN
segment.
The FN10 learns addresses from all packets, including data transmissions
and “keep alive” packets (packets sent by an idle station to let other
stations know it is present and functional). When devices are added to the
network, removed from it, or relocated, you do not have to reconfigure the
FN10. The FN10 automatically learns new device addresses, recognizes
when a previously used address is missing, or when a device has been
moved to a new LAN segment.
An address stored in the Bridge Address Table is discarded if there is no
subsequent activity from that address after a configured length of time
(five minutes by default). This aging process ensures that the Bridge
Address Table is continually updated.
Typically, addresses are continually added to and deleted from the Bridge
Address Table, reflecting the dynamic nature of internetwork traffic.
However, you can change an address from dynamic to static if you do not
want the entry in the Bridge Address Table to get discarded.
Each dynamic entry includes:
• An Ethernet MAC address

Chapter 1: Introduction
Page 1-10 Fast Network 10 User Guide
• A single port number of the LAN on which the address resides
• The age of the entry
• Various statistics counters
• Any filtering restrictions added by a Network Management Station
(NMS)
Each static entry contains the same information as a dynamic entry,
except the static entry is not aged, and can contain a range of port
numbers, rather than a single port number.
The FN10 stores 8,192 dynamic (learned) entries in its Bridge Address
Table. In addition, it stores up to 200 static or user-defined addresses.
1.5.3 FN10 Filtering
One of the most significant features of the FN10 is its user-configurable
filtering capabilities. A filter is an instruction to the FN10 to screen data
packets based on the criteria you define. Filtering is useful for gathering
statistics, implementing security measures, and improving network
performance.
The FN10 allows you to implement two types of filters that are useful for
managing and administering networks:
• Bridge Address Table filters, which use the FN10 Bridge Address
Table to screen local traffic
• Port filters, which apply filters to or from a specific port segment
See Chapter 5, FN10
Filters for instructions on setting up FN10 filters.
1.5.4 FN10 Sample Applications
Just as a six lane highway allows you to travel much faster than a single
lane highway, a network backbone creates high-speed connections for
your network. In general, a network backbone allows you to distribute
access to important network resources such as file or print servers.