Chapter 6:
Network Layer
Introduction to Networks
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Network Layer
Network Layer
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Network Layer Protocols
Network Layer in Communication
The Network layer, or OSI Layer 3, provides services to exchange the
data over the network between identified end devices. (i.e. End to End
communications)
?
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Network Layer in Communication
The Network Layer
End to End Transport processes
Addressing end devices
Encapsulation
Routing
De-encapsulating
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Network Layer in Communication
Network Layer Protocols
Common Network Layer Protocols
Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
Legacy Network Layer Protocols
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service (CLNS/DECNet)
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Characteristics of the IP protocol
Characteristics of IP
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Characteristics of the IP protocol
IP - Connectionless
IP is connectionless, it requires no initial exchange of control information to
establish an end-to-end connection, nor does it require additional fields in
the PDU header to maintain this connection.
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Characteristics of the IP protocol
IP – Best Effort Delivery
Unreliable in this context does not mean that IP works properly
sometimes and does not function well at other times.
Unreliable means simply that IP does not have the capability to manage,
and recover from, undelivered or corrupt packets. The Transport Layer
will look after reliability if required.
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Characteristics of the IP protocol
IP – Media Independent
It is the responsibility of the OSI Data Link layer to take an IP packet
and prepare it for transmission over the communications medium.
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IPv4 Packet
Encapsulating IP
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IPv4 Packet
IPv4 Packet Header
Version, Differentiated Services (DS), Time-to-Live
(TTL),Protocol, Source IP Address, Destination IP Address
Byte 1
Version
Byte 2
IP Header
Length
Byte 3
Differentiated Services
Total Length
DSCP
ECN
Identification
Time To Live
Byte 4
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
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IP V4 Packet Header
Protocol (8 bits)
This filed indicates the data payload type that the packet is carrying. The Protocol field enables the
Network layer to pass the data to the appropriate upper-layer protocol.
Example values are: 01 ICMP; 06 TCP; 17 UDP
Type-of-Service (8 bits)
The field is used to determine the priority of each packet.
This value enables a Quality-of-Service (QoS) mechanism to be applied to high priority
packets, such as those carrying telephony voice data.
Fragment Offset (13 bits)
The fragment offset field identifies the order in which to place the packet fragment in the
reconstruction.
More Fragments flag (1 bit)
The More Fragments flag bit is set (MF = 1), it means that it is not the last fragment of a packet.
Don't Fragment flag (1 bit)
If the Don't Fragment flag bit is set (DF = 1), then fragmentation of this packet is NOT permitted.
Version (4 bits) : Contains the IP version number (4).
Header Length (IHL) (4 bits) Specifies the size of the packet header.
Packet Length (16 bits) This field gives the entire packet size, including header and data, in bytes.
Identification (16 bits) This field is used for uniquely identifying fragments of an original IP packet.
Header Checksum (16 bits) The checksum field is used for error checking the packet header.
Options (variable length) There is provision for additional fields in the IPv4 header to provide other
services but these are rarely used.
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IPv4 Packet
Sample IPv4 Headers
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Network Layer in Communication
Limitations of IPv4
IP Address depletion
Internet routing table expansion
Lack of end-to-end connectivity
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Network Layer in Communication
Introducing IPv6
Increased address space
Improved packet handling
Eliminates the need for NAT
Integrated security
4 billion IPv4 addresses
232 ≈4,000,000,000
2128 ≈ 340 undecillion IPv6 addresses
340,000,000,000,000,000,000,000,000,000,000,000,000
≈ 6.67x 1027 IPv6 addresses per square meter on the planet.
Every human on planet 5x1028 IP addresses.
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Introducing IPv6
Feature
IPv6
IPv4
Easier management of networks
IPv6 networks provide autoconfiguration
capabilities. They are simpler, flatter and
more manageable, especially for large
installations.
Networks must be configured
manually or with DHCP. IPv4 has
had many overlays to handle
Internet growth, which demand
increasing maintenance efforts.
End-to-end connective integrity
Direct addressing is possible due to vast
address space - the need for network
address translation devices is effectively
eliminated.
Widespread use of NAT devices
means that a single NAT address
can mask thousands of nonroutable addresses, making end-toend integrity unachievable.
Unconstrained address
abundance
3.4 x 1038 = 340 trillion trillion trillion
addresses - about 670 quadrillion addresses
per square millimetre of the Earth's surface.
4.29 x 109 = 4.2 billion addresses far less than even a single IP
address per person on the planet.
Improved security features
IPSEC is built into the IPv6 protocol, usable
with a suitable key infrastructure.
Security is dependent on
applications - IPv4 was not
designed with security in mind.
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IPv6 Packet
Encapsulating IPv6
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Host Routing Tables
Host Packet Forwarding Decision
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Host Routing Tables
Default Gateway
Hosts must maintain their own, local, routing table to ensure
that network layer packets are directed to the correct
destination network. The local table of the host typically
contains:
Direct connection
R
Local network route
Local default route
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Host Routing Tables
Sample IPv4 Host Routing Table
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Host Routing Tables
Sample IPv6 Host Routing Table
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Router Routing Tables
Router Packet Forwarding Decision
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Router Routing Tables
IPv4 Router Routing Table
192.168.10.0/24
.10
PC1
.1
G0/1
.10
PC2
10.1.1.0/24
G0/0
.1
.10
209.165.200.224 /30
R1
.225
S0/0/0
.1
.226
R2
.1
.10
10.1.2.0/24
192.168.11.0/24
R1#show ip route
Codes: L - local, C - connected, S - static, 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
D
D
C
L
C
L
C
L
R1#
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
10.1.1.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0
10.1.2.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0
192.168.10.0/24 is variably subnetted, 2 subnets, 3 masks
192.168.10.0/24 is directly connected, GigabitEthernet0/0
192.168.10.1/32 is directly connected, GigabitEthernet0/0
192.168.11.0/24 is variably subnetted, 2 subnets, 3 masks
192.168.11.0/24 is directly connected, GigabitEthernet0/1
192.168.11.1/32 is directly connected, GigabitEthernet0/1
209.165.200.0/24 is variably subnetted, 2 subnets, 3 masks
209.165.200.224/30 is directly connected, Serial0/0/0
209.165.200.225/32 is directly connected, Serial0/0/0
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The routing
table stores
information
about
connected
and remote
networks.
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Router Routing Tables
Directly Connected Routing Table Entries
192.168.10.0/24
.10
PC1
.1
G0/1
.10
PC2
64.100.0.1
G0/0
.1
R1
.225
S0/0/0
.1
.226
R2
.1
C
.10
10.1.2.0/24
192.168.11.0/24
A
10.1.1.0/24
.10
209.165.200.224 /30
C
B
192.168.10.0/24 is directly connected, GigabitEthernet0/0
A
Identifies how the network was learned by the router.
B
Identifies the destination network and how it is connected.
C
Identifies the interface on the router connected to the destination network.
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Router Routing Tables
Remote Network Routing Table Entries
192.168.10.0/24
.10
PC1
.1
G0/1
.10
PC2
64.100.0.1
G0/0
.1
R1
.225
S0/0/0
.1
.226
R2
.1
.10
10.1.2.0/24
192.168.11.0/24
D
10.1.1.0/24
.10
209.165.200.224 /30
10.1.1.0/24 [90/2170112] via 209.165.200.226, 00:00:05, Serial0/0/0
A
Identifies how the network was learned by the router. (D=EIGRP)
B
Identifies the (remote) destination network.
C
Identifies the administrative distance (trustworthiness) of the route source.
D
Identifies the metric to reach the remote network.
E
Identifies the next hop IP address to reach the remote network.
F
Identifies the amount of elapsed time since the network was discovered.
G
Identifies the outgoing interface on the router to reach the destination network.
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Routers
Anatomy of a Router
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Anatomy of a Router
A Router is a Computer
Router CPU and OS
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Anatomy of a Router
Router Memory
Memory
Volatile /
Non-Volatile
RAM
Volatile
•
•
•
•
Running IOS
Running configuration file
IP routing and ARP tables
Packet buffer
ROM
Non-Volatile
•
•
•
Bootup instructions
Basic diagnostic software
Limited IOS
NVRAM
Non-Volatile
•
Startup configuration file
Non-Volatile
•
•
IOS
Other system files
Flash
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Stores
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Anatomy of a Router
Inside a Router
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Anatomy of a Router
Router Backplane
Double-wide eHWIC slots
eHWIC 0
AUX
port
LAN
interfaces
Console
RJ45
Two 4 GB flash card slots
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USB
Ports
Console
USB Type B
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Anatomy of a Router
LAN and WAN Interfaces
Serial interfaces (WAN links)
LAN interfaces
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Router Boot-up
Cisco IOS
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Router Boot-up
Bootset Files
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Router Boot-up
Router Bootup Process
1.Perform the POST
and load the
bootstrap program
2.Locate and load the
Cisco IOS software
3.Locate and load the
startup configuration
file or enter setup
mode
System Bootstrap, Version 15.0(1r)M15, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
<output omitted>
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Router Boot-up
Show Versions Output
Router# show version
Cisco IOS Software, C1900 Software (C1900-UNIVERSALK9-M), Version 15.2(4)M1, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2012 by Cisco Systems, Inc.
Compiled Thu 26-Jul-12 19:34 by prod_rel_team
ROM: System Bootstrap, Version 15.0(1r)M15, RELEASE SOFTWARE (fc1)
Router uptime is 10 hours, 9 minutes
System returned to ROM by power-on
System image file is "flash0:c1900-universalk9-mz.SPA.152-4.M1.bin"
Last reload type: Normal Reload
Last reload reason: power-on
<Output omitted>
Cisco CISCO1941/K9 (revision 1.0) with 446464K/77824K bytes of memory.
Processor board ID FTX1636848Z
2 Gigabit Ethernet interfaces
2 Serial(sync/async) interfaces
1 terminal line
DRAM configuration is 64 bits wide with parity disabled.
255K bytes of non-volatile configuration memory.
250880K bytes of ATA System CompactFlash 0 (Read/Write)
<Output omitted>
Technology Package License Information for Module:'c1900'
----------------------------------------------------------------Technology
Technology-package
Technology-package
Current
Type
Next reboot
-----------------------------------------------------------------ipbase
ipbasek9
Permanent
ipbasek9
security
None
None
None
data
None
None
None
Configuration register is 0x2142 (will be 0x2102 at next reload)
Router#
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Network Layer
Configuring a Cisco Router
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Configure Initial Settings
Router Configuration Steps
192.168.10.0/24
PC1
PC2
.10
G0/0
.1
.1
G0/1
.10
R1
.226
.225
S0/0/0
10.1.1.0/24
.10
209.165.200.224 /30
.1
R2
.1
.10
10.1.2.0/24
192.168.11.0/24
Router> enable
Router# configure terminal
Enter configuration commands, one per line.
End with CNTL/Z.
Router(config)# hostname R1
R1(config)#
R1(config)# enable secret class
R1(config)#
R1(config)# line console 0
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# exit
R1(config)#
R1(config)# line vty 0 4
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# exit
R1(config)#
R1(config)# service password-encryption
R1(config)#
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OR
Router> en
Router# conf t
Enter configuration commands, one per line.
End with CNTL/Z.
Router(config)# ho R1
R2(config)#
R1(config)# banner motd #
Enter TEXT message. End with the character '#'.
***********************************************
WARNING: Unauthorized access is prohibited!
***********************************************
#
R1(config)#
R1# copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
R1#
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Configure Interfaces
Configure LAN Interfaces
192.168.10.0/24
PC1
PC2
.10
10.1.1.0/24
G0/0
.1
.1
G0/1
.10
.10
209.165.200.224 /30
R1
.225
S0/0/0
.1
.226
R2
.1
.10
10.1.2.0/24
192.168.11.0/24
R1# conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#
R1(config)# interface gigabitethernet 0/0
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# description Link to LAN-10
R1(config-if)# no shutdown
%LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up
R1(config-if)# exit
R1(config)#
R1(config)# int g0/1
R1(config-if)# ip add 192.168.11.1 255.255.255.0
R1(config-if)# des Link to LAN-11
R1(config-if)# no shut
%LINK-5-CHANGED: Interface GigabitEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up
R1(config-if)# exit
R1(config)#
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Configure Interfaces
Verify Interface Configuration
192.168.10.0/24
PC1
PC2
.10
10.1.1.0/24
G0/0
.1
.1
G0/1
.10
.10
209.165.200.224 /30
R1
.1
.226
.225
S0/0/0
R2
.1
10.1.2.0/24
192.168.11.0/24
R1# show ip interface brief
Interface
IP-Address
GigabitEthernet0/0
192.168.10.1
GigabitEthernet0/1
192.168.11.1
Serial0/0/0
209.165.200.225
Serial0/0/1
unassigned
Vlan1
unassigned
R1#
R1# ping 209.165.200.226
.10
OK? Method Status
YES
YES
YES
YES
YES
manual
manual
manual
NVRAM
NVRAM
Protocol
up
up
up
up
up
up
administratively down down
administratively down down
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 209.165.200.226, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/9 ms
R1#
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Configuring a Cisco Router
Configuring the Default Gateway on PC
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Configuring the Default Gateway
Default Gateway on a Host
.10
PC1
192.168.10.0/24
.1
G0/0
.10
PC2
R1
G0/1
.1
PC4
.10
PC1
.10
PC3
192.168.11.0/24
.10
192.168.10.0/24
.1
G0/0
.11
PC2
R1
G0/1
.1
.10
PC3
.11
PC4
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192.168.11.0/24
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Configuring the Default Gateway
Default Gateway on a Switch
S1#show running-config
Building configuration...
!
<output omitted>
service password-encryption
!
hostname S1
!
Interface Vlan1
ip address 192.168.10.50
!
ip default-gateway 192.168.10.1
<output omitted>
PC1
PC2
.10
192.168.11.0/24
192.168.10.0/24
.1
G0/0
S1
.11
.50
R1
.1
G0/1
S2
If the default gateway were not configured on S1, response
packets from S1 would not be able to reach the
administrator at 192.168.11.10. The administrator would not
be able to mange the device remotely.
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