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© 2000, Cisco Systems, Inc. 3-1
Network Address Conservation
Subnetting, VLSM, NAT & RFC1918
Network Address Conservation
Subnetting, VLSM, NAT & RFC1918
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-2
AgendaAgenda
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—3-3
DefinitionsDefinitions
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—3-4
Definition: Allocation and Assignment
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—3-5
DefinitionsDefinitions
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, flap-dampened
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—3-6
Growth of Global AddressesGrowth of Global Addresses
• Growth of Global Routing Table (as of 3 May 2001)
–Unaggregated Internet would exceed 200,000 routes!
http://www.telstra.net/ops/bgptable.html
Moore’s Law and CIDRmade it work for a while
But they cannot berelied on forever
Projected routing table growth without CIDR
DeploymentPeriod of CIDR
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-7
IP Slowing IP Address DepletionIP 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—3-9
Private Addressing and Network Address Translation
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—3-10
RFC1918 - Private IP Address Ranges
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—3-11
Private Addressing and Network Address Translation
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—3-12
Types of NATTypes 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—3-13
Network Address TranslationNetwork Address Translation
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-14
Some Applications Aren't NAT-Friendly
Some Applications Aren't NAT-Friendly
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—3-15
DNS with NAT and RFC1918 Addresses
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—3-17
DefinitionsDefinitions
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—3-18
IPv4: Internet Protocol, Version 4
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—3-19
• Unique addressing allows communication between end stations
• Path choice is based on locationLocation is represented by an address
Introduction to TCP/IP Addresses
Introduction to TCP/IP Addresses
172.18.0.2
172.18.0.1
172.17.0.2172.17.0.1
172.16.0.2
172.16.0.1
SADAHDR DATA10.13.0.0 192.168.1.0
10.13.0.1 192.168.1.1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-20
IP Addressing
255 255 255 255
DottedDecimal
Maximum
Network Host
32 bits
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-21
IP AddressingIP Addressing
255 255 255 255
DottedDecimal
Maximum
Network Host
128 64 32 16 8 4 2 1
11111111 11111111 11111111 11111111Binary
32 bits
1 8 9 16 17 24 25 32
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-22
IP AddressingIP Addressing
255 255 255 255
DottedDecimal
Maximum
Network Host
128 64 32 16 8 4 2 1
11111111 11111111 11111111 11111111
10101100 00010000 01111010 11001100
Binary
32 bits
172 16 122 204ExampleDecimal
ExampleBinary
1 8 9 16 17 24 25 32
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-23
Class A:
Class B:
Class C:
Class D: Multicast
Class E: Research
IP Address ClassesIP Address Classes
NetworkNetwork HostHost HostHost HostHost
NetworkNetwork NetworkNetwork HostHost HostHost
NetworkNetwork NetworkNetwork NetworkNetwork HostHost
8 bits 8 bits 8 bits 8 bits
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-24
IP Address ClassesIP Address Classes
1
Class A:
Bits:
0NNNNNNN0NNNNNNN HostHost HostHost HostHost
8 9 16 17 24 25 32
Range (1-126)
1
Class B:
Bits:
10NNNNNN10NNNNNN NetworkNetwork HostHost HostHost
8 9 16 17 24 25 32
Range (128-191)1
Class C:
Bits:
110NNNNN110NNNNN NetworkNetwork NetworkNetwork HostHost
8 9 16 17 24 25 32
Range (192-223)1
Class D:
Bits:
1110MMMM1110MMMM Multicast GroupMulticast Group Multicast GroupMulticast Group Multicast GroupMulticast Group
8 9 16 17 2425 32
Range (224-239)
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-25
Host AddressesHost Addresses
172.16.2.1
172.16.3.10
172.16.12.12
10.1.1.1
10.250.8.11
10.180.30.118
E1
172.16 12 12
Network Host
. . Network Interface
172.16.0.0
10.0.0.0
E0
E1
Routing Table
172.16.2.1
10.6.24.2
E0
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-26
11111111
Determining Available Host Addresses
Determining Available Host Addresses
172 16 0 0
10101100 00010000 00000000 00000000
16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Network Host
00000000 00000001
11111111 11111111 11111111 11111110
...
... 00000000 00000011
11111101
123
655346553565536-
...
2
65534
N
2N-2 = 216-2 = 65534
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-27
Subnetting‑Why Subnet?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—3-28
Network 172.16.0.0
172.16.0.0
Addressing without SubnetsAddressing without Subnets
172.16.0.1 172.16.0.2 172.16.0.3
…...
172.16.255.253 172.16.255.254
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-29
Network 172.16.0.0
Addressing with SubnetsAddressing with Subnets
172.16.1.0 172.16.2.0
172.16.3.0
172.16.4.0
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-30
Subnet AddressingSubnet Addressing
172.16.2.200
172.16.2.2
172.16.2.160
172.16.2.1
172.16.3.5
172.16.3.100
172.16.3.150
E0
172.16
Network
Network Interface
172.16.0.0
172.16.0.0
E0
E1
New Routing Table
2 160
Host
. .
172.16.3.1E1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-31
Subnet AddressingSubnet Addressing
172.16.2.200
172.16.2.2
172.16.2.160
172.16.2.1
172.16.3.5
172.16.3.100
172.16.3.150
172.16.3.1
E0E1
172.16 2 160
Network Host
. . Network Interface
172.16.2.0
172.16.3.0
E0
E1
New Routing Table
Subnet
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-32
Subnet MaskSubnet Mask
172172 1616 00 00
255255 255255 00 00
255255 255255 255255 00
IPAddress
DefaultSubnet
Mask
8-bitSubnet
Mask
Network Host
Network Host
Network Subnet Host
Also written as “/16” where 16 represents the number of 1s in the mask.
Also written as “/24” where 24 represents the number of 1s in the mask.
11111111 11111111 00000000 00000000
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-33
16
Network Host
172 0 0
10101100
11111111
10101100
00010000
11111111
00010000
00000000
00000000
10100000
00000000
00000000
Subnets not in use—the default
00000010
Subnet Mask without SubnetsSubnet Mask without Subnets
172.16.2.160172.16.2.160
255.255.0.0 255.255.0.0
NetworkNumber
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-34
Network number extended by eight bits
Subnet Mask with SubnetsSubnet Mask with Subnets
16
Network Host
172.16.2.160172.16.2.160
255.255.255.0 255.255.255.0
172 2 0
10101100
11111111
10101100
00010000
11111111
00010000
11111111
00000010
10100000
00000000
00000000
00000010
Subnet
NetworkNumber
128
192
224
240
248
252
254
255
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-35
IP Host Address: 172.16.2.121Subnet Mask: 255.255.255.0
Subnet Address = 172.16.2.0Host Addresses = 172.16.2.1–172.16.2.254Broadcast Address = 172.16.2.255Eight bits of subnetting
Network Subnet Host
10101100 00010000 00000010 11111111
172.16.2.121:
255.255.255.0:
10101100
11111111
Subnet: 10101100 00010000
00010000
11111111
00000010
00000010
11111111
01111001
00000000
00000000
Class B Subnet ExampleClass B Subnet Example
Broadcast:
Network
© 2000, Cisco Systems, Inc. www.cisco.com 3-36
Variable-Length Subnet Masks
Variable-Length Subnet Masks
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-37
Variable Length Subnet Masks 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—3-38
Working with Variable Length Subnet Masks‑Subnet Design 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—3-39
Recursive Division of a Network Prefix with VLSM
Recursive Division of a Network Prefix with VLSM
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-40
Subnet MaskSubnet Mask
172172 1616 00 00
255255 255255 00 00
255255 255255 255255 00
IPAddress
DefaultSubnet
Mask
8-bitSubnet
Mask
Network Host
Network Host
Network Subnet Host
Also written as “/16” where 16 represents the number of 1s in the mask.
Also written as “/24” where 24 represents the number of 1s in the mask.
11111111 11111111 00000000 00000000
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-41
16
Network Host
172 0 0
10101100
11111111
10101100
00010000
11111111
00010000
00000000
00000000
10100000
00000000
00000000
Subnets not in use—the default
00000010
Subnet Mask without SubnetsSubnet Mask without Subnets
172.16.2.160172.16.2.160
255.255.0.0 255.255.0.0
NetworkNumber
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-42
Network number extended by eight bits
Subnet Mask with SubnetsSubnet Mask with Subnets
16
Network Host
172.16.2.160172.16.2.160
255.255.255.0 255.255.255.0
172 2 0
10101100
11111111
10101100
00010000
11111111
00010000
11111111
00000010
10100000
00000000
00000000
00000010
Subnet
NetworkNumber
128
192
224
240
248
252
254
255
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-43
Subnet Mask with Subnets (cont.)
Subnet Mask with Subnets (cont.)
Network Host
172.16.2.160172.16.2.160
255.255.255.192 255.255.255.192
10101100
11111111
10101100
00010000
11111111
00010000
11111111
00000010
10100000
11000000
10000000
00000010
Subnet
Network number extended by ten bits
16172 2 128
NetworkNumber
128
192
224
240
248
252
254
255
128
192
224
240
248
252
254
255
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-44
Decimal Equivalents of Bit Patterns
Decimal Equivalents of Bit Patterns
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
128 64 32 16 8 4 2 1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-45
VLSM Addressing ExampleVLSM Addressing Example
16172 2 160
10101100 00010000 1010000000000010 Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
4
1
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-46
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
00000010 Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-47
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
00000010 Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
7
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-48
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
10000000
00000010 Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
4
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-49
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
10000000
00000010
10111111
Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
4
56
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-50
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
10000000
00000010
10111111
10000001
Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
4
56
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-51
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
00010000
11111111 11111111
10100000
11000000
10000000
00000010
10111111
10000001
10111110
Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
4
56
7
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-52
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
10101100
00010000
11111111
00010000
11111111
00000010
10100000
11000000
10000000
00000010
10101100 00010000 00000010 10111111
10101100 00010000 00000010 10000001
10101100 00010000 00000010 10111110
Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
1
2
3
4
56
7
8
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-53
VLSM Addressing ExampleVLSM Addressing Example
10101100
11111111
10101100
00010000
11111111
00010000
11111111
00000010
10100000
11000000
10000000
00000010
10101100 00010000 00000010 10111111
10101100 00010000 00000010 10000001
10101100 00010000 00000010 10111110
Host
Mask
Subnet
Broadcast
Last
First
172.16.2.160
255.255.255.192
172.16.2.128
172.16.2.191
172.16.2.129
172.16.2.190
1
2
3
4
56
7
89
16172 2 160
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-54
IP CalculatorsIP Calculators
http://www.telusplanet.net/public/sparkman/netcalc.htm
http://www.chattanooga.net/techsupport/ipcalc/IPAddress.htm
http://ihide.virtualave.net/subnet/subnet.html
http://www.subnetonline.com/subnet/subnet.html
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-55
Address PlanningAddress 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—3-57
What Is Route Summarization?What Is Route Summarization?
Routing table172.16.25.0/24172.16.26.0/24172.16.27.0/24
172.16.27.0/24
172.16.26.0/24
172.16.25.0/24
AA
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-58
What Is Route Summarization?What Is Route Summarization?
• Routing protocols can summarize addresses of several networks into one address
I can route to the 172.16.0.0/16 network.
Routing Table172.16.0.0/16
BB
Routing Table172.16.25.0/24172.16.26.0/24172.16.27.0/24
172.16.27.0/24
172.16.26.0/24
172.16.25.0/24
AA
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-59
Summarizing Addresses in a VLSM-Designed Network
Summarizing Addresses in a VLSM-Designed Network
CorporateNetwork
172.16.0.0/16
172.16.64.0/20
172.16.128.0/20
172.16.32.64/26
172.16.32.0/24
172.16.128.0/20
172.16.32.128/26
AA
BB
CC
DD172.16.64.0/20
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-60
Route Summarization with VLSM
Route Summarization with VLSM
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-61
Summarizing within an OctetSummarizing within an Octet
172.16.168.0/24 = 10101100 . 00010000 . 10101 000 . 00000000
Number of Common Bits = 21Summary: 172.16.168.0/21
Noncommon Bits = 11
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
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-62
Benefits of Route Summarization
Benefits of Route Summarization
Increased Stability – reduce route flap through network
Reduce Router Memory Req. – smaller route tables
Reduce Router Proc. Load – smaller table
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-63
Implementation ConsiderationsImplementation 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. www.cisco.com BSCN v1.0—3-64
Route Summarization Operation in Cisco Routers
Route Summarization Operation in Cisco Routers
• Supports host-specific routes, blocks of networks, default routes
• Routers use the longest match
172.16.5.33 /32 Host172.16.5.32 /27 Subnet172.16.5.0 /24 Network172.16.0.0 /16 Block of Networks0.0.0.0 /0 Default
172.16.5.33 /32 Host172.16.5.32 /27 Subnet172.16.5.0 /24 Network172.16.0.0 /16 Block of Networks0.0.0.0 /0 Default
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-65
172.16.5.0255.255.255.0
192.168.14.16255.255.255.240
172.16.6.0255.255.255.0
Summarizing Routes in a Discontiguous Network
Summarizing Routes in a Discontiguous Network
• 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
AA BBCC
RIPv1 Will Advertise Network 172.16.0.0 RIPv1 Will Advertise
Network 172.16.0.0
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-66
172.16.5.0/24
172.16.7.0/24
192.168.14.16255.255.255.240
Be Careful When Summarizing Routes
Be Careful When Summarizing Routes
• 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
EIGRP Advertises 172.16.0.0/16 EIGRP Advertises
172.16.0.0/16
172.16.6.0/24
172.16.9.0/24AA BBCC
© 2000, Cisco Systems, Inc. www.cisco.com BSCN v1.0—3-67
Route Summarization Overview
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