Network Address
Conservation
Subnetting, VLSM,
NAT & RFC1918
© 2000, Cisco Systems, Inc.
3-1
Agenda
Need for Address Conservation
Private Addressing and NAT
Classful Addressing
Variable-Length Subnet Masks
Route Aggregation
Summary
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-2
Definitions
Regional Internet Registry (RIR)
– An organization with regional responsibility for
management of Internet resources
– Responsibilities include allocation/registration
services, coordination and policy development
– For example. APNIC, ARIN, RIPE-NCC
Local Internet Registry (LIR)
– Otherwise known as an ARIN Member
– Usually operates as an ISP, assigns address space
to its customers and registers it in the ARIN
database
• Eg. NJ Edge, UUNET
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-3
Definition: Allocation and
Assignment
RFC 2050 – Allocation Guidelines
http://sunsite.dk/RFC/rfc/rfc2050.html
Allocation
• A block of address space held by an IR for
subsequent allocation or assignment
• Not yet used to address any networks
Assignment
• A block of address space used to address an
operational network
• May be provided to LIR customers, or used for an
LIR’s infrastructure (‘self-assignment’)
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-4
Definitions
Provider Independent (Portable)
– Customer holds addresses independent from ISP
– Customer keeps addresses when changing ISP
– Bad for size of routing tables
– Bad for QOS: routes may be filtered, flapdampened
Provider Aggregatable (Non-portable)
– Customer uses ISP’s address space
– Customer must renumber if changing ISP
– Only way to effectively scale the Internet
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-5
Growth of Global Addresses
• Growth of Global Routing Table (as of 3 May 2001)
– Unaggregated Internet would exceed 200,000
routes!
Projected routing table
growth without CIDR
But they cannot be
relied on forever
Moore’s Law and CIDR
made it work for a while
Deployment
Period of CIDR
© 2000, Cisco Systems, Inc.
http://www.telstra.net/ops/bgptable.html
www.cisco.com
BSCN v1.0—3-6
IP Slowing IP Address Depletion
• Subnet masking; RFCs 950, 1812
• Address allocation for private Internets,
RFC 1918
• Network Address Translation (NAT), RFC
1631
• Hierarchical addressing
• Variable-length subnet masks (VLSM), RFC
1812
• Route summarization, RFC 1518
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-7
Private Addresses
and NAT
© 2000, Cisco Systems, Inc.
www.cisco.com
3-8
Private Addressing and Network
Address Translation
One way to cope with the depletion of IP
addresses is through the use of private
addressing.
IP addresses used on the Internet must be
globally unique, usually specified by an
Internet service provider.
However, traffic that remains only on an
organization's private network does not need
to be globally unique, just unique across that
organization's private network.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-9
RFC1918 - Private IP Address
Ranges
Used for networks/hosts not on Internet
• Class A:
1; 10.0.0.0 ~ 10.255.255.255
• Class B: 16; 172.16.0.0 ~ 172.31.255.255
• Class C: 256; 192.168.0.0 ~ 192.168.255.255
Planning:
• Determine which hosts are internal ONLY
• Routers configured with filters
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-10
Private Addressing and Network
Address Translation
RFC1918 Private Addresses are not
routed on the Internet.
Host Computers using Private IP address
space can still send and receive traffic
to/from the Internet by using RFC 1631
network address translation (NAT).
NAT can be provided by a router, firewall,
or stand-alone NAT software running
on a multi-homed server.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-11
Types of NAT
Static NAT – direct mapping of inside address
to outside address, one to one correlation
Dynamic NAT – outside address pulled from
pool of addresses when needed then released
back to pool when no longer needed, likely
different address each time
PAT (Port Address Translation) – Special type
of dynamic NAT where pool consists of one
address, every host appears to internet as the
same address, differentiated by source port
number (also called Address Overloading)
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-12
Network Address Translation
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-13
Some Applications Aren't NATFriendly
Some applications send IP addresses or port
numbers hidden inside their datapackets,
where NAT can't properly rewrite them - so
those applications don't work when you try to
use them on computers behind NATs.
Breaks Global Addressing – problem for peer
to peer networking (like napster, netmeeting,
etc)
DNS needs special handling in large
environments
Additional Info:
http://sunsite.dk/RFC/rfc/rfc1631.html
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-14
DNS with NAT and RFC1918
Addresses
Two DNS Servers may be needed, one to resolve
internal names with Internal Addresses and the
another to maintain your DNS domain to the
Internet. Both DNS servers must be independent
each other, so that all Internal computers must
point to your Internal DNS, and your Internal DNS
could be configured with a forwarder pointing to
the Internet DNS server that will help you to
resolve the rest of Internet names.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-15
Classful
Addressing
© 2000, Cisco Systems, Inc.
www.cisco.com
3-16
Definitions
Classful and Classless
• Classful
–Address architecture where network
boundaries are fixed at 8, 16 or 24 bits
(class A, B, and C)
• Classless
–Architecture in which network boundaries
may occur at any bit (e.g. /12, /16, /19, /24
etc)
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-17
IPv4: Internet Protocol,
Version 4
IP address is 32-bit, binary, 4-octets
Dotted-decimal format for human consumption
Address space divided into classes (A~E)
• A: 1.h.h.h ~ 126.h.h.h, 16.7M hosts
• B: 128.1.h.h ~191.254.h.h, 65K hosts
• C: 192.0.1.h ~ 223.255.254.h, 254 hosts
• D: 224.0.0.0 ~ 239.255.255.254, Multicasting
• E: 240.0.0.0 ~ 255.255.255.255, IETF Research
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-18
Introduction to TCP/IP
Addresses
172.18.0.1
172.18.0.2
10.13.0.0
10.13.0.1
172.16.0.1
HDR SADA DATA
172.17.0.1
172.16.0.2
172.17.0.2
192.168.1.0
192.168.1.1
• Unique addressing allows communication
between end stations
• Path choice is based on location
Location is represented by an address
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-19
IP Addressing
32 bits
Dotted
Decimal
Maximum
© 2000, Cisco Systems, Inc.
Network
255
255
www.cisco.com
Host
255
255
BSCN v1.0—3-20
IP Addressing
32 bits
Dotted
Decimal
Network
© 2000, Cisco Systems, Inc.
16 17
255
24 25
32
11111111 11111111
11111111 11111111
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
8 9
255
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
1
Binary
255
255
Maximum
Host
www.cisco.com
BSCN v1.0—3-21
IP Addressing
32 bits
Dotted
Decimal
Network
16 17
255
24 25
32
11111111 11111111
11111111 11111111
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
8 9
255
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
1
Binary
255
255
Maximum
Host
Example
172
16
122
204
Decimal
Example 10101100 00010000 01111010 11001100
Binary
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-22
IP Address Classes
8 bits
8 bits
8 bits
8 bits
Host
Host
Host
Host
Host
Class A:
Network
Class B:
Network Network
Class C:
Network Network Network
Class D:
Multicast
Class E:
Research
© 2000, Cisco Systems, Inc.
www.cisco.com
Host
BSCN v1.0—3-23
IP Address Classes
Bits:
Class A:
Bits:
Class B:
Bits:
Class C:
Bits:
Class D:
© 2000, Cisco Systems, Inc.
1
8 9
0NNNNNNN
16 17
24 25
Host
Host
32
Host
Range (1-126)
1
8 9
10NNNNNN
16 17
Network
Range (128-191)
1
8 9
110NNNNN
Host
16 17
Network
Range (192-223)
1
8 9
1110MMMM
24 25
Host
24 25
Network
16 17
32
32
Host
24 25
32
Multicast Group Multicast Group Multicast Group
Range (224-239)
www.cisco.com
BSCN v1.0—3-24
Host Addresses
172.16.2.1
10.1.1.1
10.6.24.2
E1
172.16.3.10
E0
172.16.2.1
10.250.8.11
172.16.12.12
172.16
Network
© 2000, Cisco Systems, Inc.
.
12 . 12
Host
10.180.30.118
Routing Table
Network
Interface
172.16.0.0
E0
10.0.0.0
E1
www.cisco.com
BSCN v1.0—3-25
Determining Available Host
Addresses
Network
0
0
© 2000, Cisco Systems, Inc.
...
...
10101100 00010000 00000000 00000000
00000000 00000001
00000000 00000011
N
1
2
3
...
16
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
172
Host
11111111 11111101
11111111 11111110
11111111 11111111
65534
65535
65536
2
2N-2 = 216-2 = 65534
65534
www.cisco.com
BSCN v1.0—3-26
Subnetting-Why Subnet?
Address classes were restrictive and forced an
inefficient allocation of addresses. (Class C
too small but Class B too large). Class B
addresses were given out to organizations
that would never need the 65,534 addresses.
RFC 950, defined in 1985, provided a way to
subnet or provide a third layer of
organization or hierarchy between the
existing network ID and the existing host ID.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-27
Addressing without Subnets
172.16.0.1 172.16.0.2 172.16.0.3
172.16.255.253 172.16.255.254
…...
172.16.0.0
Network 172.16.0.0
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-28
Addressing with Subnets
172.16.3.0
172.16.4.0
172.16.1.0
172.16.2.0
Network 172.16.0.0
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-29
Subnet Addressing
172.16.2.200
172.16.3.5
172.16.3.1
E1
172.16.2.2
E0
172.16.2.1
172.16.3.100
172.16.2.160
172.16
Network
© 2000, Cisco Systems, Inc.
.
172.16.3.150
2 . 160
New Routing Table
Network
Interface
Host
172.16.0.0
E0
172.16.0.0
E1
www.cisco.com
BSCN v1.0—3-30
Subnet Addressing
172.16.2.200
172.16.3.5
172.16.3.1
E1
E0
172.16.2.1
172.16.2.2
172.16.3.100
172.16.2.160
172.16
Network
© 2000, Cisco Systems, Inc.
.
2
172.16.3.150
.
160
Subnet Host
New Routing Table
Network
Interface
172.16.2.0
E0
172.16.3.0
E1
www.cisco.com
BSCN v1.0—3-31
Subnet Mask
Network
IP
Address
172
Host
16
0
Network
Default
Subnet
Mask
8-bit
Subnet
Mask
© 2000, Cisco Systems, Inc.
255
0
Host
255
0
0
11111111
11111111
00000000
00000000
Also written as “/16” where 16 represents the number of 1s
in the mask.
Network
Subnet
Host
255
255
255
0
Also written as “/24” where 24 represents the number of 1s
in the mask.
www.cisco.com
BSCN v1.0—3-32
Subnet Mask without Subnets
Network
Host
172.16.2.160
10101100
00010000
00000010
10100000
255.255.0.0
11111111
11111111
00000000
00000000
10101100
00010000
00000000
00000000
172
16
0
0
Network
Number
Subnets not in use—the default
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-33
Subnet Mask with Subnets
Network
172.16.2.160
Host
10101100
00010000
00000010
10100000
11111111
11111111
11111111
00000000
10101100
00010000
00000010
00000000
172
16
128
192
224
240
248
252
254
255
255.255.255.0
Subnet
Network
Number
2
0
Network number extended by eight bits
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-34
Class B Subnet Example
IP Host Address: 172.16.2.121
Subnet Mask: 255.255.255.0
Network
Network
Subnet
Host
172.16.2.121: 10101100
00010000
00000010
01111001
255.255.255.0: 11111111
11111111
11111111
00000000
Subnet: 10101100
00010000
00000010
00000000
Broadcast: 10101100
00010000
00000010
11111111
Subnet Address = 172.16.2.0
Host Addresses = 172.16.2.1–172.16.2.254
Broadcast Address = 172.16.2.255
Eight bits of subnetting
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-35
Variable-Length
Subnet Masks
© 2000, Cisco Systems, Inc.
www.cisco.com
3-36
Variable Length Subnet Masks
Variable Length Subnet Masks (VLSM), defined
in 1987 as RFP 1009. A single network ID
could have different subnet masks among its
subnets.
The major benefit of VLSM is that subnets can
be defined to different sizes as needed under
a single Network ID, thereby minimizing, if
not eliminating, wasted addresses.
Second, variable length subnet masks can be
used to permit route aggregation which
minimizes the number of distinct routes that
need to be advertised and processed by
network backbone or Internet routers.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-37
Working with Variable Length
Subnet Masks-Subnet Design
Subnet design with VLSM is similar to subnet
design with fixed length masks except that
decisions made regarding subnets are
made independently at each level in the
VLSM scenario. At each level two questions
must be answered:
1. How many subnets are required at this level both
now and in the future?
2. What is the largest number of hosts required per
subnet on this level both now and in the future?
The answers to these questions will determine
how many subnets with how much host ID
capacity needs to be defined at each level.
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-38
Recursive Division of a Network Prefix
with VLSM
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-39
Subnet Mask
Network
IP
Address
172
Host
16
0
Network
Default
Subnet
Mask
8-bit
Subnet
Mask
© 2000, Cisco Systems, Inc.
255
0
Host
255
0
0
11111111
11111111
00000000
00000000
Also written as “/16” where 16 represents the number of 1s
in the mask.
Network
Subnet
Host
255
255
255
0
Also written as “/24” where 24 represents the number of 1s
in the mask.
www.cisco.com
BSCN v1.0—3-40
Subnet Mask without Subnets
Network
Host
172.16.2.160
10101100
00010000
00000010
10100000
255.255.0.0
11111111
11111111
00000000
00000000
10101100
00010000
00000000
00000000
172
16
0
0
Network
Number
Subnets not in use—the default
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-41
Subnet Mask with Subnets
Network
172.16.2.160
Host
10101100
00010000
00000010
10100000
11111111
11111111
11111111
00000000
10101100
00010000
00000010
00000000
172
16
128
192
224
240
248
252
254
255
255.255.255.0
Subnet
Network
Number
2
0
Network number extended by eight bits
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-42
Subnet Mask with Subnets
(cont.)
255.255.255.192
Network
Number
Host
10101100
00010000
00000010
10100000
11111111
11111111
11111111
11000000
10101100
00010000
00000010
10000000
128
192
224
240
248
252
254
255
172.16.2.160
Subnet
128
192
224
240
248
252
254
255
Network
172
16
2
128
Network number extended by ten bits
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-43
Decimal Equivalents of Bit
Patterns
128 64
© 2000, Cisco Systems, Inc.
32
16
8
4
2
1
1
0
0
0
0
0
0
0
=
128
1
1
0
0
0
0
0
0
=
192
1
1
1
0
0
0
0
0
=
224
1
1
1
1
0
0
0
0
=
240
1
1
1
1
1
0
0
0
=
248
1
1
1
1
1
1
0
0
=
252
1
1
1
1
1
1
1
0
=
254
1
1
1
1
1
1
1
1
=
255
www.cisco.com
BSCN v1.0—3-44
VLSM Addressing Example
172.16.2.160
172
16
10101100
00010000
2
160
00000010 10100000 Host
1
Mask
255.255.255.192
Subnet 4
Broadcast
First
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-45
VLSM Addressing Example
172.16.2.160
255.255.255.192
172
16
10101100
00010000
11111111
11111111
2
160
00000010 10100000 Host
1
11111111 11000000 Mask 2
Subnet
Broadcast
First
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-46
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
11111111
11111111
00000010 10100000 Host
1
11111111 11000000 Mask 2
Subnet
Broadcast
First
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
7
BSCN v1.0—3-47
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
11111111
11111111
00000010 10100000 Host
1
11111111 11000000 Mask 2
10000000 Subnet 4
Broadcast
First
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-48
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
11111111
11111111
00000010 10100000 Host
1
11111111 11000000 Mask 2
10000000 Subnet 4
10111111 Broadcast
5
First
6
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-49
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
11111111
11111111
00000010 10100000 Host
1
11111111 11000000 Mask 2
10000000 Subnet 4
10111111 Broadcast
5
10000001 First
6
Last
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-50
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
11111111
11111111
00000010 10100000 Host
1
11111111 11000000 Mask 2
10000000 Subnet 4
10111111 Broadcast
© 2000, Cisco Systems, Inc.
www.cisco.com
5
10000001 First
6
10111110 Last
7
BSCN v1.0—3-51
VLSM Addressing Example
172
16
2
160
3
172.16.2.160
255.255.255.192
10101100
00010000
00000010 10100000 Host
11111111
11111111
11111111 11000000 Mask 2
10101100
00010000
00000010 10000000 Subnet 4
10101100
00010000
00000010 10111111 Broadcast
10101100
00010000
5
00000010 10000001 First
6
10101100
00010000
00000010 10111110 Last
7
1
8
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-52
VLSM Addressing Example
172
16
2
160
3
10101100
00010000
255.255.255.192 11111111
8
9
172.16.2.128
10101100
11111111
11111111 11000000 Mask 2
00010000
00000010 10000000 Subnet 4
10101100
00010000
00000010 10111111 Broadcast
6
7
172.16.2.160
172.16.2.191
00000010 10100000 Host
172.16.2.129
10101100
00010000
5
00000010 10000001 First
172.16.2.190
10101100
00010000
00000010 10111110 Last
© 2000, Cisco Systems, Inc.
www.cisco.com
1
BSCN v1.0—3-53
IP Calculators
http://www.telusplanet.net/public/sparkman/net
calc.htm
http://www.chattanooga.net/techsupport/ipcalc/
IPAddress.htm
http://ihide.virtualave.net/subnet/subnet.html
http://www.subnetonline.com/subnet/subnet.ht
ml
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-54
Address Planning
• Map IP Addressing Scheme to Physical
Topology or Logical Groups
• Anticipate Growth!
• Leave ‘spare’ Subnets
• Restrict Size of Subnets
• Deploy Address blocks with
Summarization in mind
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-55
Route
Summarization
© 2000, Cisco Systems, Inc.
www.cisco.com
3-56
What Is Route Summarization?
172.16.25.0/24
172.16.26.0/24
A
172.16.27.0/24
Routing table
172.16.25.0/24
172.16.26.0/24
172.16.27.0/24
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-57
What Is Route Summarization?
172.16.25.0/24
I can route to the
172.16.0.0/16 network.
172.16.26.0/24
A
B
172.16.27.0/24
Routing Table
172.16.25.0/24
172.16.26.0/24
172.16.27.0/24
Routing Table
172.16.0.0/16
• Routing protocols can summarize addresses of several
networks into one address
© 2000, Cisco Systems, Inc.
www.cisco.com
BSCN v1.0—3-58
Summarizing Addresses in a
VLSM-Designed Network
172.16.128.0/20
B
172.16.32.64/26
172.16.32.0/24
A
C
172.16.0.0/16
172.16.32.128/26
172.16.64.0/20
© 2000, Cisco Systems, Inc.
Corporate
Network
D
www.cisco.com
BSCN v1.0—3-59
Route Summarization
with VLSM
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BSCN v1.0—3-60
Summarizing within an Octet
172.16.168.0/24 = 10101100 . 00010000 . 10101 000 . 00000000
172.16.169.0/24 =
172
.
16
. 10101 001 .
0
172.16.170.0/24 =
172
.
16
. 10101 010 .
0
172.16.171.0/24 =
172
.
16
. 10101 011 .
0
172.16.172.0/24 =
172
.
16
. 10101 100 .
0
172.16.173.0/24 =
172
.
16
. 10101 101 .
0
172.16.174.0/24 =
172
.
16
. 10101 110 .
0
172.16.175.0/24 =
172
.
16
. 10101 111 .
0
Number of Common Bits = 21
Summary: 172.16.168.0/21
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Noncommon
Bits = 11
BSCN v1.0—3-61
Benefits of Route
Summarization
Increased Stability – reduce route
flap through network
Reduce Router Memory Req. –
smaller route tables
Reduce Router Proc. Load – smaller
table
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BSCN v1.0—3-62
Implementation Considerations
• Multiple IP addresses must have the
same highest-order bits
• Routing decisions are made based
on the entire address
• Routing protocols must carry the
prefix (subnet mask) length
© 2000, Cisco Systems, Inc.
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BSCN v1.0—3-63
Route Summarization
Operation in Cisco Routers
172.16.5.33
172.16.5.32
172.16.5.0
172.16.0.0
0.0.0.0
/32
/27
/24
/16
/0
Host
Subnet
Network
Block of Networks
Default
• Supports host-specific routes, blocks of
networks, default routes
• Routers use the longest match
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BSCN v1.0—3-64
Summarizing Routes in a
Discontiguous Network
172.16.5.0
255.255.255.0
192.168.14.16
255.255.255.240
A
C
RIPv1 Will Advertise
Network 172.16.0.0
172.16.6.0
255.255.255.0
B
RIPv1 Will Advertise
Network 172.16.0.0
• RIPv1 and IGRP do not advertise subnets, and
therefore cannot support discontiguous subnets
• OSPF, EIGRP, and RIPv2 can advertise subnets,
and therefore can support discontiguous
subnets
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BSCN v1.0—3-65
Be Careful When Summarizing
Routes
192.168.14.16
255.255.255.240
172.16.5.0/24
172.16.7.0/24
172.16.6.0/24
A
C
EIGRP Advertises
172.16.0.0/16
B
172.16.9.0/24
EIGRP Advertises
172.16.0.0/16
• EIGRP on both Router A and Router B advertise
a summarized route to 172.16.0.0/16
• Router C receives two routes to 172.16.0.0/16
• Router A (or B or both) should be configured to
not summarize
© 2000, Cisco Systems, Inc.
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BSCN v1.0—3-66
Route Summarization
Overview
•Synonymous with aggregation or supernetting
•Minimizes routing table entries
•Isolates topology changes from other routers
•Summary of MSB to LSB
•Most effective when network addresses are
contiguous
•Most effective when network addressing uses
VLSM and is hierarchical
•Common bits determined from MSB to LSB
•Can occur at each layer of a scalable network
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BSCN v1.0—3-67
Questions?
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3-68