Understanding The Routing
Table
Part 1: The Routing Table Structure
Part 2: The Lookup Process
Rick Graziani
Cabrillo College
The Routing Table on PDF
•
Instructors and Students: Complete information
explanations associated with this PowerPoint can be found
on my web site.
Understanding the Routing Table
• The Routing Table: Part 1 or 2 - The Routing Table
Structure (PDF)
• The Routing Table: Part 2 or 2 - The Routing Table Lookup
Process (PDF)
• Discard Routes - Avoiding Routing Loops (PowerPoint)
Rick Graziani graziani@cabrillo.edu
2
Part 1: The Routing Table
Structure
Directly Connected Networks
172.16.1.0/24
172.16.3.0/24
.1 fa0
172.16.4.0/24
.1 fa0
.1
172.16.2.0/24
s0
.1 fa0
.2
.1
s0
s1
Router A
Router B
192.168.1.0/24
.2
s0
Router C
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter
area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
RouterB#
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Directly Connected Networks
RouterB#debug ip routing
RouterB(config)#interface s 1
RouterB(config-if)#ip add 192.168.1.1 255.255.255.0
RouterB(config-if)#no shutdown
00:59:48: %LINK-3-UPDOWN: Interface Serial1, changed state to up
00:59:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state
to up
00:59:48: RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0]
00:59:48: RT: interface Serial1 added to routing table
RouterB(config-if)#end
RouterB#undebug all
All possible debugging has been turned off
RouterB#
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Directly Connected Networks
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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Creating a Parent/Child Route
RouterB(config)#interface fastethernet 0
RouterB(config-if)#ip add 172.16.3.1 255.255.255.0
RouterB(config-if)#no shutdown
01:21:11: RT: add 172.16.3.0/24 via 0.0.0.0, connected metric [0/0]
01:21:11: RT: interface FastEthernet0 added to routing table
RouterB(config-if)#end
RouterB#undebug all
All possible debugging has been turned off
RouterB#
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 1 subnets
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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Parent, Child, Ultimate Routes
172.16.1.0/24
172.16.3.0/24
.1 fa0
172.16.4.0/24
.1 fa0
172.16.2.0/24
.1
s0
Router A
.1 fa0
.2
.1
s0
s1
Router B
192.168.1.0/24
.2
s0
Router C
So far on Router B:
Parent Route:
Child Route:
Ultimate Route:
C
C
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172.16.0.0/24 is subnetted, 1 subnets
172.16.3.0 is directly connected, FastEthernet0
192.168.1.0/24 is directly connected, Serial1
8
Creating Another Child Route
RouterB(config)#inter s 0
RouterB(config-if)#ip add 172.16.2.2 255.255.255.0
RouterB(config-if)#end
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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9
Same structure no matter the source of
route
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 4 subnets
S
172.16.4.0 is directly connected, Serial1
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
10.0.0.0/16 is subnetted, 1 subnets
S
10.1.0.0 is directly connected, Serial1
C
192.168.1.0/24 is directly connected, Serial1
S
192.168.100.0/24 is directly connected, Serial1
RouterB#
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10
Static Routes and Recursive Lookups
172.16.1.0/24
.1 fa0
.1
172.16.2.0/24
s0
172.16.3.0/24
172.16.4.0/24
.1 fa0
.1 fa0
.2
.1
s0
s1
Router A
Router B
192.168.1.0/24
.2
s0
Router C
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C
172.16.1.0 is directly connected, FastEthernet0
C
172.16.2.0 is directly connected, Serial0
RouterA#
RouterA(config)#ip route 172.16.3.0 255.255.255.0 172.16.2.2
03:12:45: RT: add 172.16.3.0/24 via 172.16.2.2, static metric [1/0]
RouterA(config)#
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Static Routes and Recursive Lookups
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24
C
172.16.1.0
C
172.16.2.0
S
172.16.3.0
RouterA#
is subnetted, 3 subnets
is directly connected, FastEthernet0
is directly connected, Serial0
[1/0] via 172.16.2.2
First lookup of 172.16.3.0:
S
172.16.3.0 [1/0] via 172.16.2.2
Second lookup of 172.16.2.0:
C
172.16.2.0 is directly connected, Serial0
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Static Routes without Recursive Lookup
172.16.1.0/24
.1 fa0
.1
172.16.2.0/24
s0
172.16.3.0/24
172.16.4.0/24
.1 fa0
.1 fa0
.2
.1
s0
s1
Router A
192.168.1.0/24
Router B
.2
s0
Router C
RouterB(config)#ip route 172.16.1.0 255.255.255.0 serial 0
05:05:30: RT: add 172.16.1.0/24 via 0.0.0.0, static metric [1/0]
RouterB(config)#
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
S
172.16.1.0 is directly connected, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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13
Static Routes and Administrative Distance
You might have also noticed that the routing table states that this static route is “directly connected.”
S
172.16.1.0 is directly connected, Serial0
• In the case of a static route, directly connected means that the static
•
•
•
•
•
route was configured with an exit interface.
This does not mean that it is a directly connected network like an
interface on the router.
Like all static routes, the default administrative distance of this static
route is still “1”.
Only directly connected interfaces, (which have a code of “C” in the
routing table) can have an administrative distance of “0”.
Although it is possible to modify the administrative distance of static
and dynamic routes, they cannot be given the administrative distance
of “0”.
Likewise, directly connected interfaces can only have an administrative
distance of “0”.
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Static Routes via Ethernet Interfaces
172.16.1.0/24
172.16.3.0/24
.1 fa0
172.16.4.0/24
.1 fa0
.1
172.16.2.0/24
fa1
.1 fa0
.2
.1
fa1
s1
Router A
192.168.1.0/24
.2
s0
Router B
Router C
RouterB(config)#ip route 172.16.1.0 255.255.255.0 172.16.2.1
RouterB(config)#end
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
S
172.16.1.0/24 [1/0] via 172.16.2.1
C
172.16.2.0 is directly connected, FastEthernet1
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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Static Routes via Ethernet Interfaces
RouterB(config)#ip route 172.16.1.0 255.255.255.0 fastethernet 1 172.16.2.1
RouterB(config)#end
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
S
172.16.1.0/24 [1/0] via 172.16.2.1 FastEthernet1
C
172.16.2.0 is directly connected, FastEthernet1
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
RouterB#
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VLSM
172.16.3.0/24
.1 fa0
172.16.1.4/30
.5
.9
s0
s1
172.16.1.8/30
Router X
RouterX#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/16 is
C
172.16.1.4/30
C
172.16.1.8/30
C
172.16.3.0/24
RouterX#
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variably subnetted, 3 subnets, 2 masks
is directly connected, Serial0
is directly connected, Serial1
is directly connected, FastEthernet0
17
Part 2: The Lookup Process
Subnet Mask = # of bits that must match
Figure 2
IP Packet
192.168.1.10
11000000
10101000
00000001
00001010
16 bits must match, but only one bit matches.
Parent
Route
172.16.0.0
10101100
00010000
00000000
00000000
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C
172.16.1.0 is directly connected, FastEthernet0
C
172.16.2.0 is directly connected, Serial0
R
172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0
R
192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0
RouterA#
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Subnet Mask = # of bits that must match
Figure 3
IP Packet
192.168.1.10
11000000
10101000
00000001
00001010
These 24 bits match!
Ultimate
Route
192.168.1.0
11000000
10101000
00000001
00000000
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C
172.16.1.0 is directly connected, FastEthernet0
C
172.16.2.0 is directly connected, Serial0
R
172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0
R
192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0
RouterA#
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Parent then Children
IP Packet
172.16.3.10
10101100
00010000
00000011
00001010
00000000
00000000
These 16 bits match!
Parent
Route
172.16.0.0
10101100
00010000
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C
172.16.1.0 is directly connected, FastEthernet0
C
172.16.2.0 is directly connected, Serial0
R
172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0
R
192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0
RouterA#
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Figure 6 (child routes shown in different order than with ‘show ip route’)
IP Packet
172.16.3.10
10101100
00010000
00000011
00001010
These 24 bits match!
Child
Route
172.16.3.0
10101100
00010000
00000011
00000000
Child
Route
172.16.1.0
10101100
00010000
00000001
00000000
Child
Route
172.16.2.0
10101100
00010000
00000010
00000000
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C
172.16.1.0 is directly connected, FastEthernet0
C
172.16.2.0 is directly connected, Serial0
R
172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0
R
192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0
RouterA#
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The Default Route
RouterB(config)#ip route 0.0.0.0 0.0.0.0 serial1
RouterB(config)#end
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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23
Classful Routing Behavior
RouterB#show running-config
Building configuration...
Current configuration:
!
version 11.2
!
<text omitted>
!
no ip classless
!
<text omitted>
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24
Classful Routing Behavior
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
No match!
Child
Route
172.16.3.0
10101100
00010000
00000011
00000000
Child
Route
172.16.1.0
10101100
00010000
00000001
00000000
Child
Route
172.16.2.0
10101100
00010000
00000010
00000000
RouterB#show ip route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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25
Classful Routing Behavior
Routing Behavior: classful (no ip classless)
Packet’s Destination IP Address: 172.16.4.10
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
<text omitted>
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
1.
2.
3.
4.
Match
No Match
No Match
No Match
5. Drop Packet
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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26
Classless Routing Behavior
RouterB#show running-config
Building configuration...
Current configuration:
!
version 11.3
!
<text omitted>
!
ip classless
!
<text omitted>
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27
Classless Routing Behavior
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
No match!
Child
Route
172.16.3.0
10101100
00010000
00000011
00000000
Child
Route
172.16.1.0
10101100
00010000
00000001
00000000
Child
Route
172.16.2.0
10101100
00010000
00000010
00000000
RouterB#show ip route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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28
Classless Routing Behavior
Routing Behavior: classless (ip classless)
Packet’s Destination IP Address: 172.16.4.10
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
<text omitted>
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
1.
2.
3.
4.
Match
No Match
No Match
No Match
5. Match!
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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29
Classless and Classful Routing Behavior
Changing the default routing behavior, whether classful or classless, is simple. The command to enable
classless routing behavior is:
RouterB(config)#ip classless
RouterB(config)#
The command to disable classless routing behavior, in other words, enable classful routing behavior is:
RouterB(config)#no ip classless
RouterB(config)#
Use show running-config to verify which command is in affect.
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30
/16 Classful Routes
172.16.1.0/24
.1 fa0
.1
172.16.2.0/24
s0
172.16.3.0/24
172.16.4.0/24
.1 fa0
.1 fa0
.2
.1
s0
s1
192.168.1.0/24
172.16.10.0/24 lo1
.2
172.16.20.0/24 lo2
s0
172.16.30.0/24 lo3
172.16.40.0/24 lo4
RouterC(config)#interface
RouterC(config-if)#ip add
RouterC(config-if)#exit
RouterC(config)#interface
RouterC(config-if)#ip add
RouterC(config-if)#exit
loopback1
172.20.0.1 255.255.0.0
loopback2
172.30.0.1 255.255.0.0
RouterC#show ip inter brief
Interface
IP-Address
FastEthernet0
172.16.4.1
Loopback1
172.20.0.1
Loopback2
172.30.0.1
Serial0
192.168.1.2
RouterC#
Rick Graziani graziani@cabrillo.edu
OK?
YES
YES
YES
YES
Method
NVRAM
manual
manual
NVRAM
Status
up
up
up
up
Protocol
up
up
up
up
31
/16 Classful Routes
172.16.1.0/24
.1 fa0
.1
172.16.2.0/24
s0
172.16.3.0/24
172.16.4.0/24
.1 fa0
.1 fa0
.2
.1
s0
s1
192.168.1.0/24
172.16.10.0/24 lo1
.2
172.16.20.0/24 lo2
s0
172.16.30.0/24 lo3
172.16.40.0/24 lo4
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C
172.16.4.0/24 is directly connected, FastEthernet0
S
172.16.0.0/16 is directly connected, Serial0
C
172.20.0.0/16 is directly connected, Loopback1
C
172.30.0.0/16 is directly connected, Loopback2
C
192.168.1.0/24 is directly connected, Serial0
RouterC#
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32
Static Routes to Supernets
RouterB(config)#ip route 172.0.0.0 255.0.0.0 serial1
RouterB(config)#ip route 160.0.0.0 240.0.0.0 serial1
RouterB(config)#end
RouterB#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<text omitted>
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S
172.0.0.0/8 is directly connected, Serial1
S
160.0.0.0/4 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
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33
Supernets
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
No match!
Child
Route
172.16.3.0
10101100
00010000
00000011
00000000
Child
Route
172.16.1.0
10101100
00010000
00000001
00000000
Child
Route
172.16.2.0
10101100
00010000
00000010
00000000
RouterB#show ip route
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S
172.0.0.0/8 is directly connected, Serial1
S
160.0.0.0/4 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
Rick Graziani graziani@cabrillo.edu
34
Supernets – 172.0.0.0 best match
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
Longest-bit-match! (11 bits match, 8 bits minimum)
Supernet
172.0.0.0/8
10101100
00000000
00000000
00000000
Supernet
160.0.0.0/4
10100000
00000000
00000000
00000000
Default
0.0.0.0/0
00000000
00000000
00000000
00000000
RouterB#show ip route
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S
172.0.0.0/8 is directly connected, Serial1
S
160.0.0.0/4 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
Rick Graziani graziani@cabrillo.edu
35
Supernets – 160.0.0.0 best match
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
Longest-bit-match! (4 bits)
Supernet
160.0.0.0/4
10100000
00000000
00000000
00000000
Default
0.0.0.0/0
00000000
00000000
00000000
00000000
RouterB#show ip route
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S
160.0.0.0/4 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
Rick Graziani graziani@cabrillo.edu
36
The last of the Supernets – 0.0.0.0/0
IP Packet
172.16.4.10
10101100
00010000
00000100
00001010
Match! Zero bits needed to match, which still makes it a
match!
Default
0.0.0.0/0
00000000
00000000
00000000
00000000
RouterB#show ip route
172.16.0.0/24 is subnetted, 3 subnets
R
172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0
C
172.16.2.0 is directly connected, Serial0
C
172.16.3.0 is directly connected, FastEthernet0
C
192.168.1.0/24 is directly connected, Serial1
S*
0.0.0.0/0 is directly connected, Serial1
RouterB#
Rick Graziani graziani@cabrillo.edu
37
The Routing Table on PDF
•
Complete information explanations associated with this
PowerPoint can be found on my web site.
Understanding the Routing Table
• The Routing Table: Part 1 or 2 - The Routing Table
Structure (PDF)
• The Routing Table: Part 2 or 2 - The Routing Table Lookup
Process (PDF)
• Discard Routes - Avoiding Routing Loops (PowerPoint)
Rick Graziani graziani@cabrillo.edu
38