Avaya IP, ARP, RARP, RIP, and OSPF User Manual
BayRS Version 15.1
Part No. 308627-15.1 Rev 00
October 2001
600 Technology Park Drive
Billerica, MA 01821-4130
Configuring IP, ARP, RARP, RIP, and OSPF Services
Copyright © 2001 Nortel Networks
All rights reserved. October 2001.
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308627-15.1 Rev 00
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308627-15.1 Rev 00
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Contents
Preface
Before You Begin ............................................................................................................ xvii
Text Conventions ........................................................................................................... xviii
Acronyms .........................................................................................................................xix
Related Publications ........................................................................................................xxi
How to Get Help .............................................................................................................xxii
Chapter 1
IP Concepts, Terminology, and Features
IP Addresses .................................................................................................................. 1-2
Subnet Addressing ...................................................................................................1-4
Supernet Addressing ...............................................................................................1-7
Classless Interdomain Routing ................................................................................ 1-8
Autonomous Systems .....................................................................................................1-8
Addresses and Variable-Length Masks ........................................................................1-10
Address Resolution Protocol (ARP) .............................................................................1-10
Reverse Address Resolution Protocol (RARP) .............................................................1-13
Routing Information Protocol (RIP) ...............................................................................1-14
Open Shortest Path First (OSPF) Protocol ...................................................................1-14
OSPF Neighbors ....................................................................................................1-15
Neighbor Adjacencies ............................................................................................1-15
Designated Routers ...............................................................................................1-16
AS External Routes ................................................................................................1-16
OSPF Areas ...........................................................................................................1-17
Stub Area .........................................................................................................1-18
Not-So-Stubby Area (NSSA) ...........................................................................1-18
OSPF Router Types ...............................................................................................1-19
OSPF Implementation Notes .................................................................................1-19
Router Discovery Protocol ............................................................................................1-20
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v
Route Preferences ........................................................................................................ 1-21
Route Weights ..............................................................................................................1-22
IP Routing Policies and Filters ......................................................................................1-23
How OSPF Implements Routing Policies ...............................................................1-27
IP Traffic Filters .............................................................................................................1-28
Chapter 2
Starting IP Services
Using the BCC ................................................................................................................ 2-2
Starting IP ................................................................................................................2-2
Step 1: Configuring a Physical Interface ............................................................2-2
Step 2: Configuring an IP Interface ....................................................................2-3
Starting RIP ..............................................................................................................2-3
Starting OSPF ..........................................................................................................2-4
Starting Router Discovery ........................................................................................2-4
Using Site Manager ........................................................................................................2-5
Starting IP ................................................................................................................2-5
Starting RIP ..............................................................................................................2-6
Adding RIP to an IP Interface ............................................................................2-7
Starting OSPF ..........................................................................................................2-8
Starting RARP ..........................................................................................................2-9
Starting Router Discovery ......................................................................................2-10
Chapter 3
Configuring and Customizing IP
Customizing IP Global Parameters .................................................................................3-2
Disabling and Reenabling Global IP ........................................................................ 3-3
Configuring the Router for Not-Forwarding Mode .................................................... 3-4
Specifying Whether to Listen for RIP Packets in Host-Only Mode ..................... 3-5
Configuring Bridging on a Router in Not-Forwarding Mode ...............................3-6
Setting the Time-to-Live Value on a Source Packet ................................................. 3-9
Allowing an All-Zero or All-One Subnet Address ...................................................3-10
Estimating the Size of the Routing Table ................................................................3-11
Using a Default Route for Unknown Subnets .........................................................3-13
Specifying the Maximum Number of IP Policies .....................................................3-14
Setting the ICMP Error Limit .................................................................................. 3-15
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Disabling and Reenabling Route Filter Support .....................................................3-16
Configuring Equal-Cost Multipath Support for RIP and OSPF ...............................3-17
Enabling ISP Mode on the Router ..........................................................................3-20
Customizing the IP Routing Table Structure ..........................................................3-22
Specifying the Percentage of Buffers Available to ARP .........................................3-23
Enabling Directed Broadcast on a Router ..............................................................3-24
Blocking Source Record Routing ..................................................................................3-25
Customizing an IP Interface ..........................................................................................3-26
Disabling and Reenabling an IP Interface .............................................................. 3-29
Specifying a Broadcast Address for an Interface ...................................................3-30
Specifying the Cost of an Interface ........................................................................3-31
Enabling MTU Discovery on an Interface ...............................................................3-33
Enabling and Disabling ICMP Address-Mask Replies ............................................3-34
Disabling and Reenabling ICMP Redirect Messages .............................................3-36
Enabling All-Subnet Broadcasting on an Interface .................................................3-37
Disabling UDP Checksum Processing on an Interface ..........................................3-39
Specifying a MAC Address ....................................................................................3-40
Enabling Source Routing over a Token Ring Network ............................................3-41
Configuring the MTU for an IP Interface .................................................................3-44
Configuring an SMDS Address ..............................................................................3-45
Configuring a WAN Address for a Frame Relay Network .......................................3-46
Specifying the Maximum Size of the Forwarding Table ..........................................3-47
Deleting IP from an Interface .................................................................................3-49
Configuring the Circuitless IP Interface ........................................................................3-50
Choosing Slots to Support the Circuitless Interface ...............................................3-52
Configuring an Unnumbered IP Interface .....................................................................3-53
Creating an Unnumbered Interface ........................................................................ 3-55
Using the Alternate Associated Address Option ....................................................3-56
Configuring an Adjacent Host Address .........................................................................3-57
Defining a Static Route .................................................................................................3-60
Defining a Default Route ........................................................................................3-64
Defining a Static Black Hole for a Supernet ...........................................................3-64
Customizing Router Discovery .....................................................................................3-65
Enabling and Disabling Router Discovery .............................................................. 3-66
Choosing a Broadcast Type ...................................................................................3-67
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Specifying the Time Interval Between Advertisements ..........................................3-68
Configuring the Lifetime of Advertised Addresses ................................................. 3-69
Specifying Interface Preference .............................................................................3-70
Chapter 4
Customizing ARP and RARP Services
Customizing ARP ...........................................................................................................4-2
Disabling and Reenabling Global ARP .................................................................... 4-2
Customizing Global ARP ..........................................................................................4-3
Logging Nonlocal ARP Source Addresses ........................................................4-3
Setting the Nonlocal ARP Destination Response .............................................. 4-3
Selecting an Address Resolution Scheme for an IP Interface .................................. 4-5
Selecting an Encapsulation Option for ARP and Probe ...........................................4-8
Enabling Proxy ARP on an Interface ........................................................................4-9
Timing Out Entries in the Address Resolution Cache ............................................ 4-11
Customizing RARP .......................................................................................................4-13
Disabling and Reenabling RARP on an Interface ..................................................4-13
Defining the RARP Mapping Table .........................................................................4-14
Disabling and Reenabling RARP Globally .............................................................4-15
Deleting RARP Globally .........................................................................................4-16
Chapter 5
Customizing RIP Services
Setting the RIP Diameter ................................................................................................5-2
Customizing a RIP Interface ...........................................................................................5-3
Disabling and Reenabling RIP on an Interface ........................................................ 5-3
Selecting the RIP Version ........................................................................................5-4
Authenticating the Password on a Version 2 Update ................................................ 5-6
Supplying RIP Updates on an Interface ................................................................... 5-8
Specifying the Update Mode ....................................................................................5-9
Sending Triggered Updates ....................................................................................5-11
Specifying a Time-to-Live Value .............................................................................5-12
Receiving RIP Updates ..........................................................................................5-14
Supplying a Default Route on an Interface .............................................................5-15
Listening for a Default Route ..................................................................................5-17
Configuring a RIP Interface for Dial-Optimized Routing .........................................5-18
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Specifying the Full Update Interval ..................................................................5-18
Specifying a Timeout Period ............................................................................5-20
Specifying a Holddown Period .........................................................................5-21
Specifying a Stabilization Time ........................................................................5-22
Specifying Frame Relay Services for Adjacent Hosts ............................................5-23
Deleting RIP from an Interface ...............................................................................5-24
Configuring RIP Accept and Announce Policies ...........................................................5-25
Defining a RIP Accept Policy ..................................................................................5-26
Specifying Match Criteria for a RIP Accept Policy .................................................. 5-29
Supplying Modification Values for a RIP Accept Policy .......................................... 5-31
Defining a RIP Announce Policy ............................................................................ 5-32
Specifying Match Criteria for a RIP Announce Policy ............................................5-35
Supplying Modification Values for a RIP Announce Policy .....................................5-40
Chapter 6
Customizing OSPF Services
Customizing OSPF Global Parameters ..........................................................................6-2
Disabling and Reenabling OSPF on the Router .......................................................6-3
Supplying an OSPF Router ID .................................................................................6-4
Configuring the Soloist on a Slot ..............................................................................6-5
Configuring an OSPF Router as a Boundary Router ...............................................6-6
Configuring the Metric Type for an ASE Advertisement .....................................6-8
Choosing a Tag Generation Method for an ASE Advertisement ......................6-11
Setting the Holddown Timer ................................................................................... 6-14
Setting a Maximum Number of Refreshes for Summary and ASE LSAs ...............6-15
Configuring Message Logging ...............................................................................6-16
Configuring External Route Preference .................................................................6-18
Customizing OSPF on an IP Interface ..........................................................................6-19
Disabling and Reenabling OSPF on an Interface ...................................................6-20
Changing the Area ID .............................................................................................6-21
Specifying the Interface Type .................................................................................6-22
Specifying Router Priority for a Multiaccess Network ............................................6-24
Estimating the Transit Delay ...................................................................................6-26
Setting the Retransmit Interval ...............................................................................6-27
Setting the Hello Interval ........................................................................................6-28
Setting the Dead Interval .......................................................................................6-30
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Setting the Poll Interval for NBMA Neighbors ........................................................6-32
Specifying the Metric Cost .....................................................................................6-33
Specifying the MTU Size ........................................................................................6-35
Specifying the MTU Mismatch Detect ....................................................................6-37
Configuring a Neighbor on an NBMA Interface ......................................................6-38
Deleting OSPF from an IP Interface .......................................................................6-41
Configuring Areas .........................................................................................................6-42
Creating an Area ....................................................................................................6-42
Disabling and Reenabling an Area .........................................................................6-44
Configuring Authentication ..................................................................................... 6-45
Configuring a Summary Route ...............................................................................6-47
Configuring a Stub Area .........................................................................................6-49
Configuring a Not-So-Stubby Area (NSSA) ...........................................................6-51
Configuring an NSSA Address Range ..........................................................................6-55
Configuring an Area Border Router ..............................................................................6-58
Configuring a Virtual Link Through a Transit Area .................................................6-59
Configuring OSPF Accept and Announce Policies .......................................................6-64
Defining an OSPF Accept Policy ............................................................................6-65
Specifying Match Criteria for an OSPF Accept Policy ............................................6-68
Defining an OSPF Announce Policy .......................................................................6-70
Specifying Match Criteria for an OSPF Announce Policy .......................................6-72
Specifying Modification Values for an OSPF Announce Policy ..............................6-76
Appendix A
Site Manager Parameters
IP Parameters ................................................................................................................ A-2
IP Configuration Parameters ................................................................................... A-2
IP Interface Parameters .......................................................................................... A-5
IP Global Parameters ............................................................................................ A-18
Static Route Parameters ....................................................................................... A-27
Adjacent Host Parameters .................................................................................... A-30
Router Discovery Parameters ............................................................................... A-33
RARP Parameters ....................................................................................................... A-35
OSPF Parameters ....................................................................................................... A-36
OSPF Global Parameters ...................................................................................... A-37
OSPF Interface Parameters .................................................................................. A-41
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Neighbor Parameters for an NBMA Interface ........................................................ A-47
OSPF Area Parameters ........................................................................................ A-49
Area Range Parameters ........................................................................................ A-53
OSPF NSSA Range Parameters ........................................................................... A-56
OSPF Virtual Interface Parameters ....................................................................... A-59
RIP Parameters ........................................................................................................... A-62
Accept and Announce Policy Parameters .................................................................... A-69
Common Accept Policy Parameters ...................................................................... A-70
RIP Accept Policy Parameters .............................................................................. A-73
OSPF Accept Policy Parameters ........................................................................... A-75
Common Announce Policy Parameters ................................................................. A-76
RIP Announce Policy Parameters ......................................................................... A-86
OSPF Announce Policy Parameters ..................................................................... A-87
Appendix B
Import and Export Route Filters
RIP Import Filters .......................................................................................................... B-2
RIP Export Filters .......................................................................................................... B-5
OSPF Import Filters ....................................................................................................... B-8
OSPF Export Filters .................................................................................................... B-11
Appendix C
Route Weight Worksheet
Appendix D
IP/OSPF Configuration Example
Configuring Internal Backbone Router 1 ....................................................................... D-3
Configuring Area Border Router 2 ................................................................................. D-4
Configuring Area Border Router 3 ................................................................................. D-6
Configuring Area Border Router 4 ................................................................................. D-8
Configuring Internal Backbone Router 5 ..................................................................... D-10
Configuring AS Boundary Router 6 ............................................................................. D-11
Index
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xi
Figures
Figure 1-1. Network and Host Portions of IP Addresses ............................................ 1-3
Figure 1-2. Supernet Addresses ................................................................................1-7
Figure 1-3. Internetwork Segmented into Three Autonomous Systems .....................1-9
Figure 1-4. ARP Example .........................................................................................1-11
Figure 1-5. RARP Server Supplying an IP Address .................................................1-13
Figure 1-6. OSPF Areas ........................................................................................... 1-17
Figure 1-7. IP Routing Table .....................................................................................1-24
Figure 1-8. Accept and Announce Policies ...............................................................1-26
Figure 3-1. IP Interface .............................................................................................3-27
Figure 3-2. IP Routers Source Routing Across a Token Ring Network ....................3-42
Figure 4-1. Proxy ARP Example ................................................................................4-9
Figure 6-1. OSPF ASE Routes ...................................................................................6-9
Figure 6-2. AS External Route Tag ...........................................................................6-12
Figure 6-3. Example of Using Configurable Cost Metrics ........................................6-33
Figure 6-4. Area Border Router ................................................................................ 6-58
Figure 6-5. Virtual Link and Transit Area ..................................................................6-60
Figure D-1. IP/OSPF Configuration ........................................................................... D-2
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xiii
Tables
Table 1-1. Network Classes and IP Addresses .........................................................1-2
Table 1-2. Subnet Masks for Class B and Class C Addresses .................................1-6
Table 3-1. Source Routing Bridge Support for Host-Only Mode ............................... 3-6
Table 3-2. Learning Bridge Support for Host-Only Mode ..........................................3-7
Table 3-3. MAC Address Parameter Settings .........................................................3-40
Table 3-4. BCC Adjacent Host Parameters .............................................................3-58
Table 3-5. Required BCC Static Route Parameters ................................................3-62
Table 3-6. Optional BCC Static Route Parameters .................................................3-62
Table 5-1. RIP Version Options .................................................................................5-5
Table 5-2. BCC Definition Parameters for RIP Accept Policies ...............................5-27
Table 5-3. BCC Match Parameters for RIP Accept Policies ....................................5-29
Table 5-4. BCC Definition Parameters for RIP Announce Policies .........................5-33
Table 5-5. BCC Match Parameters for RIP Announce Policies ...............................5-35
Table 5-6. BCC Modification Parameters for RIP Announce Policies .....................5-40
Table 6-1. OSPF Log Messages .............................................................................6-16
Table 6-2. OSPF Interface Types ............................................................................ 6-23
Table 6-3. OSPF Retransmit Interval Settings ........................................................6-27
Table 6-4. OSPF Hello Interval Settings .................................................................6-29
Table 6-5. OSPF Dead Interval Settings .................................................................6-30
Table 6-6. OSPF Metric Cost Settings (per RFC 2328) ..........................................6-34
Table 6-7. BCC Neighbor Parameters for an NBMA Interface ................................6-39
Table 6-8. BCC Parameters for OSPF NSSA .........................................................6-53
Table 6-9. BCC Address Range Parameters for OSPF NSSA ...............................6-56
Table 6-10. BCC Parameters for OSPF Virtual Link .................................................6-62
Table 6-11. BCC Definition Parameters for OSPF Accept Policies ...........................6-66
Table 6-12. BCC Match Parameters for OSPF Accept Policies ................................ 6-68
Table 6-13. BCC Definition Parameters for OSPF Announce Policies ......................6-71
Table 6-14. BCC Match Parameters for OSPF Announce
Policies 6-73
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Table 6-15. BCC Modification Parameters for OSPF Announce Policies ................. 6-76
Table D-1. Internal Backbone Router 1 .................................................................... D-3
Table D-2. Area Border Router 2 .............................................................................. D-4
Table D-3. Area Border Router 3 .............................................................................. D-6
Table D-4. Area Border Router 4 .............................................................................. D-8
Table D-5. Internal Backbone Router 5 .................................................................. D-10
Table D-6. AS Boundary Router 6 .......................................................................... D-11
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This guide describes Internet Protocol (IP) services and what you do to start and
customize IP services on a Nortel Networks* router.
You can use the Bay Command Console (BCC*) or Site Manager to configure IP
services on a router. In this guide, you will find instructions for using both the
BCC and Site Manager.
Before You Begin
Before using this guide, you must complete the following procedures. For a new
router:
Preface
• Install the router (see the installation guide that came with your router).
• Connect the router to the network and create a pilot configuration file (see
Make sure that you are running the latest version of Nortel Networks BayRS* and
Site Manager software. For information about upgrading BayRS and Site
Manager, see the upgrading guide for your version of BayRS.
308627-15.1 Rev 00
Quick-Starting Routers, Configuring BayStack Remote Access, or Connecting
ASN Routers to a Network).
xvii
Configuring IP, ARP, RARP, RIP, and OSPF Services
Text Conventions
This guide uses the following text conventions:
angle brackets (< >) Indicate that you choose the text to enter based on the
description inside the brackets. Do not type the
brackets when entering the command.
Example: If the command syntax is:
ping
<ip_address>
ping 192.32.10.12
, you enter:
bold text
Indicates command names and options and text that
you need to enter.
Example: Enter
Example: Use the
show ip {alerts | routes
dinfo
command.
}.
braces ({}) Indicate required elements in syntax descriptions
where there is more than one option. You must choose
only one of the options. Do not type the braces when
entering the command.
Example: If the command syntax is:
show ip {alerts | routes
show ip alerts or show ip routes
}
, you must enter either:
, but not both.
brackets ([ ]) Indicate optional elements in syntax descriptions. Do
not type the brackets when entering the command.
Example: If the command syntax is:
show ip interfaces [-alerts
show ip interfaces
or
]
, you can enter either:
show ip interfaces -alerts
.
ellipsis points (. . . ) Indicate that you repeat the last element of the
command as needed.
xviii
Example: If the command syntax is:
ethernet/2/1
ethernet/2/1
[<parameter> <value>]
and as many parameter-value pairs as
needed.
. . .
, you enter
308627-15.1 Rev 00
Preface
italic text Indicates new terms, book titles, and variables in
command syntax descriptions. Where a variable is two
or more words, the words are connected by an
underscore.
Example: If the command syntax is:
show at
valid_route
<valid_route>
is one variable and you substitute one value
for it.
screen text Indicates system output, for example, prompts and
system messages.
Acronyms
Example:
Set Trap Monitor Filters
separator ( > ) Shows menu paths.
Example: Protocols > IP identifies the IP option on the
Protocols menu.
vertical line (
) Separates choices for command keywords and
|
arguments. Enter only one of the choices. Do not type
the vertical line when entering the command.
Example: If the command syntax is:
show ip {alerts | routes
show ip alerts
or
}
show ip routes
This guide uses the following acronyms:
ARE all routes explorer
ARP Address Resolution Protocol
AS autonomous system
, you enter either:
, but not both.
308627-15.1 Rev 00
ASB all-subnet broadcast
ASBR AS boundary router
ASE autonomous system external
ATM asynchronous transfer mode
xix
Configuring IP, ARP, RARP, RIP, and OSPF Services
BGP Border Gateway Protocol
BootP Bootstrap Protocol
BR border router
CIDR classless interdomain routing
DVMRP Distance Vector Multicast Routing Protocol
ECMP equal-cost multipath
EGP Exterior Gateway Protocol
FDDI Fiber Distributed Data Interface
FIFO first in first out
ICMP Internet Control Message Protocol
IGP interior gateway protocol
IP Internet Protocol
ISP Internet service provider
LIS logical IP subnet
LSA link state advertisement
xx
LSDB link state database
MAC media access control
MOSPF Multicasting Open Shortest Path First
MTU maximum transmission unit
NBMA nonbroadcast multi-access
NIC Network Information Center
NSSA not-so-stubby area
OSPF Open Shortest Path First
PPP Point-to-Point Protocol
RARP Reverse Address Resolution Protocol
RFC Request for Comments
RIF routing information field
RIP Routing Information Protocol
SMDS Switched Multimegabit Data Service
308627-15.1 Rev 00
SNMP Simple Network Management Protocol
SRR source record route
STE spanning tree explorer
TTL time to live
UDP User Datagram Protocol
VC virtual circuit
WAN wide area network
Related Publications
For more information about IP services, refer to the following publications:
• Reference for BCC IP show Commands (part number 308603-14.20 Rev 00)
Provides descriptions of all
commands that display IP, ARP, RIP, and OSPF configuration and statistical
data.
show
commands for IP services, including the
Preface
• Configuring IP Exterior Gateway Protocols (BGP and EGP) (part number
• Configuring GRE, NAT, RIPSO, and BFE Services (part number
You can print selected technical manuals and release notes free, directly from the
Internet. Go to the www.nortelnetworks.com/documentation URL. Find the
product for which you need documentation. Then locate the specific category and
model or version for your hardware or software product. Use Adobe* Acrobat
Reader* to open the manuals and release notes, search for the sections you need,
and print them on most standard printers. Go to Adobe Systems at the
www.adobe.com URL to download a free copy of the Adobe Acrobat Reader.
308627-15.1 Rev 00
308628-15.0 Rev 00)
Provides a description of Border Gateway Protocol (BGP) and Exterior
Gateway Protocol (EGP) services and instructions for configuring them.
308625-14.20 Rev 00)
Provides a description of Generic Routing Encapsulation (GRE), Network
Address Translation (NAT), Revised IP Security Option (RIPSO), and Blacker
front-end services and instructions for configuring them.
xxi
Configuring IP, ARP, RARP, RIP, and OSPF Services
You can purchase selected documentation sets, CDs, and technical publications
through the Internet at the www1.fatbrain.com/documentation/nortel/ URL.
How to Get Help
If you purchased a service contract for your Nortel Networks product from a
distributor or authorized reseller, contact the technical support staff for that
distributor or reseller for assistance.
If you purchased a Nortel Networks service program, contact one of the following
Nortel Networks Technical Solutions Centers:
Technical Solutions Center Telephone
Europe, Middle East, and Africa (33) (4) 92-966-968
North America (800) 4NORTEL or (800) 466-7835
Asia Pacific (61) (2) 9927-8800
China (800) 810-5000
xxii
An Express Routing Code (ERC) is available for many Nortel Networks products
and services. When you use an ERC, your call is routed to a technical support
person who specializes in supporting that product or service. To locate an ERC for
your product or service, go to the http://www130.nortelnetworks.com/cgi-bin/
eserv/common/essContactUs.jsp
URL.
308627-15.1 Rev 00
Chapter 1
IP Concepts, Terminology, and Features
This chapter introduces concepts and terminology used in this guide.
Topic Page
IP Addresses
Autonomous Systems 1-8
Addresses and Variable-Length Masks 1-10
Address Resolution Protocol (ARP) 1-10
Reverse Address Resolution Protocol (RARP) 1-13
Routing Information Protocol (RIP) 1-14
Open Shortest Path First (OSPF) Protocol 1-14
Router Discovery Protocol 1-20
Route Preferences 1-21
Route Weights 1-22
IP Routing Policies and Filters 1-23
IP Traffic Filters 1-28
1-2
308627-15.1 Rev 00
1-1
Configuring IP, ARP, RARP, RIP, and OSPF Services
IP Addresses
An IP address consists of 32 bits that have the form network.host. The network
portion is a network number ranging from 8 to 24 bits. The host portion is the
remaining 8 to 24 bits identifying a specific host on the network. The Internet
Network Information Center (NIC) assigns the network portion of the IP address.
Your network administrator assigns the host portion.
NIC recognizes three primary classes of networks: A, B, and C. In addition, NIC
has recently identified two other classes: Class D for networks that support
multicasting, which allows an IP datagram to be transmitted to a single multicast
group consisting of hosts spread across separate physical networks; and Class E
for experimental networks. The IP router does not fully support Class D or Class E
networks.
Based on the size of the network, NIC classifies a network as Class A, B, or C (the
most common). The network class determines the number of bits assigned to the
network and host portions of the IP address (Tab le 1- 1
Table 1-1. Network Classes and IP Addresses
).
1-2
Network Size Class Network Portion Host Portion
More than 65,533 hosts A 8 bits 24 bits
254 to 65,533 hosts B 16 bits 16 bits
Fewer than 254 hosts C 24 bits 8 bits
The position of the first bit set to 0 (whether it is the first, second, third, or fourth
bit) in the first octet of an IP address indicates the network class (A, B, C, or D). If
no bit is set to 0, it is a Class E network. Figure 1-1
shows the placement of the
first bit set to 0 for Class A, B, and C networks. The figure also shows how a
network’s class affects the network and host portions of the IP address.
308627-15.1 Rev 00
IP Concepts, Terminology, and Features
8162431
Class A
Class B
Class C
Class A
Class B
Class C
0
8162431
0
1
8162431
101
First Octet Network Host
0
10
110
Network portion
Range
1-127
128-191
192-223
Host portion
Example
25.0.0.1
140.250.0.1
192.2.3.1
25
140.250
192.2.3
Figure 1-1. Network and Host Portions of IP Addresses
1
1
1
IP0005A
You specify IP addresses in dotted-decimal notation. To express an IP address in
dotted-decimal notation, you convert each 8-bit octet of the IP address to a
decimal number and separate the numbers by decimal points.
For example, you specify the 32-bit IP address 10000000 00100000 00001010
10100111 in dotted-decimal notation as 128.32.10.167. The most significant 2
bits (10) in the first octet indicate that the network is Class B; therefore, the first
16 bits compose the NIC-assigned network portion field. The third octet
(00001010) and fourth octet (10100111) compose the host field.
308627-15.1 Rev 00
1-3
Configuring IP, ARP, RARP, RIP, and OSPF Services
Subnet Addressing
The concept of subnetworks (or subnets) extends the IP addressing scheme.
Subnets are two or more physical networks that share a common
network-identification field (the NIC-assigned network portion of the 32-bit IP
address). Subnets allow an IP router to hide the complexity of multiple LANs
from the rest of the internetwork.
Using subnets, you partition the host portion of an IP address into a subnet
number and a “real” host number on that subnet. The IP address is then defined by
network.subnet.host. Routers outside the network do not interpret the subnet and
host portions of the IP address separately.
Routers within a network containing subnets use a 32-bit subnet mask that
identifies the extension bits. In network.subnet.host, the subnet.host portion (or
the local portion) contains an arbitrary number of bits. The network administrator
allocates bits within the local portion to subnet and host, and then assigns values
to subnet and host.
For example, the following is the IP address of a network that contains
subnets: 10000000 00100000 00001010 10100111. You specify this address in
dotted-decimal notation as 128.32.10.167.
1-4
The second bit of the first octet is set to 0, indicating that the network is a Class B
network. Therefore, the NIC-assigned network portion contains 16 bits and the
locally assigned local portion contains 16 bits.
The network administrator allocates the 16 bits in the local portion field as
follows:
• Upper 8 bits (00001010) with a value of 10 to the subnet portion
• Lower 8 bits (10100111) with a value of 167 to the host portion
In other words, the 16-bit local portion field, together with the 16-bit network
field, specify host 167 on subnet 10 of network 128.32.
You now need a subnet mask to identify those bits in the 32-bit IP address that
specify the network field and those bits that specify the subnet field. Like the IP
address, you specify the subnet mask in dotted-decimal notation.
308627-15.1 Rev 00
IP Concepts, Terminology, and Features
You construct a subnet mask as follows:
• Assign a value of 1 to each of the 8, 16, or 24 bits in the network field.
• Assign a value of 1 to each bit in the subnet field.
• Assign a value of 0 to each bit in the host field.
• Convert the resulting 32-bit string to dotted-decimal notation.
For example, to construct a subnet mask for the IP address 10000000 00100000
00001010 10100111, do the following:
1. Assign a value of 1 to each bit in the network field.
The position of the first bit set to 0 in the first octet of the IP address indicates
that the network is Class B; therefore, the network field contains 16 bits:
11111111 11111111.
2. Assign a value of 1 to each bit in the subnet field.
Allocate the upper 8 bits of the local portion field to the subnet portion, as
follows: 11111111.
3. Assign a value of 0 to each bit in the host field.
4. Convert the resulting 32-bit string (11111111 11111111 11111111 00000000)
308627-15.1 Rev 00
Allocate the lower 8 bits of the local portion field to the host identification, as
follows: 00000000.
to dotted-decimal notation, as follows: 255.255.255.0.
1-5
Configuring IP, ARP, RARP, RIP, and OSPF Services
Tabl e 1-2 lists the range of possible subnet masks for Class B and Class C
addresses, along with the number of bits that the mask allocates for a subnet
address, the number of recommended subnets associated with the mask, and the
number of hosts per subnet.
Table 1-2. Subnet Masks for Class B and Class C Addresses
Number of Subnets
Number of Bits Subnet Mask
Class B
2 255.255.192.0 2 16,382
3 255.255.224.0 6 8190
4 255.255.240.0 14 4094
5 255.255.248.0 30 2046
6 255.255.252.0 62 1022
7 255.255.254.0 126 510
8 255.255.255.0 254 254
9 255.255.255.128 510 126
10 255.255.255.192 1,022 62
11 255.255.255.224 2,046 30
12 255.255.255.240 4,094 14
13 255.255.255.248 8,190 6
14 255.255.255.252 16,382 2
Class C
2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 6
6 255.255.255.252 62 2
(Recommended)
Number of Hosts
per Subnet
1-6
308627-15.1 Rev 00
Supernet Addressing
A supernet is a group of networks identified by contiguous network addresses.
Internet service providers (ISPs) can assign customers blocks of contiguous
addresses to define supernets as needed.
Each supernet has a unique supernet address that consists of the upper bits shared
by all addresses in the contiguous block. For example, Figure 1-2
block of contiguous 32-bit addresses (192.32.0.0 through 192.32.7.0 in
dotted-decimal notation).
11000000 00100000 00000000 00000000
11000000 00100000 00000001 00000000
11000000 00100000 00000010 00000000
11000000 00100000 00000011 00000000
11000000 00100000 00000100 00000000
11000000 00100000 00000101 00000000
11000000 00100000 00000110 00000000
11000000 00100000 00000111 00000000
IP Concepts, Terminology, and Features
illustrates a
Figure 1-2. Supernet Addresses
The supernet address for this block is 11000000 00100000 00000, the 21 upper
bits shared by the 32-bit addresses.
A complete supernet address consists of an address/mask pair:
• address is the first 32-bit IP address in the contiguous block. In this example,
• mask is a 32-bit string containing a set bit for each bit position in the supernet
The complete supernet address in this example is 192.32.0.0/255.255.248.0.
308627-15.1 Rev 00
IP0007A
the address is 11000000 00100000 00000000 00000000 (192.32.0.0 in
dotted-decimal notation).
part of the address. The mask for the supernet address in this example is
11111111 11111111 11111000 00000000 (255.255.248.0 in dotted-decimal
notation).
1-7
Configuring IP, ARP, RARP, RIP, and OSPF Services
Classless Interdomain Routing
Classless interdomain routing (CIDR) is an addressing scheme that employs
supernet addresses to represent multiple IP destinations. Rather than advertise a
separate route for each destination in a supernet, a router can use a supernet
address to advertise a single route — called an aggregate route — that represents
all of the destinations. CIDR reduces the size of the routing tables used to store
advertised IP routes.
OSPF supports classless routing within a domain.
Autonomous Systems
LANs and WANs interconnected by IP routers form a group of networks called an
internetwork. For administrative purposes, an internetwork is divided into
autonomous systems. An autonomous system (AS) is simply a collection of
routers (called gateways in IP terminology) and hosts run by a single technical
administrator that has a single, clearly defined routing policy. Each autonomous
system has its own unique AS number assigned by the appropriate Internet
Registry entity. Figure 1-3
autonomous systems.
depicts a sample internetwork segmented into three
1-8
308627-15.1 Rev 00
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