The Network Layer - MODULO 8

Question 1

What network layer provides

Answer 1

The network layer, or OSI Layer 3, provides services to allow end devices to exchange data
across networks.

Question 2

Who are principle network layer protocols

Answer 2

, IP version 4 (IPv4) and IP version 6 (IPv6)

Question 3

What is others protocols

Answer 3

Other network layer protocols include routing protocols such as Open Shortest Path First
(OSPF) and messaging protocols such as Internet Control Message Protocol (ICMP).

Question 4

To accomplish end-to-end communications across network boundaries, network layer
protocols perform four basic operations:

Answer 4

Addressing end devices
Encapsulation
Routing
De-encapsulation

Question 5

Addressing end devices

Answer 5

End devices must be configured with a unique IP
address for identification on the network.

Question 6

Encapsulation

Answer 6

The network layer encapsulates the protocol data unit (PDU) from
the transport layer into a packet. The encapsulation process adds IP header
information, such as the IP address of the source (sending) and destination
(receiving) hosts. The encapsulation process is performed by the source of the IP
packet.

Question 7

Routing

Answer 7

The network layer provides services to direct the packets to a destination
host on another network. To travel to other networks, the packet must be processed
by a router. The role of the router is to select the best path and direct packets
toward the destination host in a process known as routing. A packet may cross
many routers before reaching the destination host. Each router a packet crosses to
reach the destination host is called a hop.

Question 8

De-encapsulation

Answer 8

When the packet arrives at the network layer of the destination
host, the host checks the IP header of the packet. If the destination IP address
within the header matches its own IP address, the IP header is removed from the
packet. After the packet is de-encapsulated by the network layer, the resulting
Layer 4 PDU is passed up to the appropriate service at the transport layer. The de
encapsulation process is performed by the destination host of the IP packet.

Question 9

NOTES

Answer 9

Unlike the transport layer (OSI Layer 4), which manages the data transport between the
processes running on each host, network layer communication protocols (i.e., IPv4 and
IPv6) specify the packet structure and processing used to carry the data from one host to
another host. Operating without regard to the data carried in each packet allows the
network layer to carry packets for multiple types of communications between multiple hosts.

Question 10

Describes IP encapsulation

Answer 10

IP encapsulates the transport layer (the layer just above the network layer) segment or
other data by adding an IP header. The IP header is used to deliver the packet to the
destination host.
The figure illustrates how the transport layer PDU is encapsulated by the network layer
PDU to create an IP packet.

Question 11

The process of encapsulating affects the other layers

Answer 11

The process of encapsulating data layer by layer enables the services at the different
layers to develop and scale without affecting the other layers.
This means the transport layer segments can be readily packaged by IPv4 or IPv6 or by
any new protocol that might be developed in the future.
The IP header is examined by Layer 3 devices (i.e., routers and Layer 3 switches) as it
travels across a network to its destination.
It is important to note, that the IP addressing information remains the same from the time
the packet leaves the source host until it arrives at the destination host, except when
translated by the device performing Network Address Translation (NAT) for IPv4.
Note: NAT is discussed in later modules.
Routers implement routing protocols to route packets between networks.
The routing performed by these intermediary devices examines the network layer
addressing in the packet header.
In all cases, the data portion of the packet, that is, the encapsulated transport layer PDU or
other data, remains unchanged during the network layer processes.

Question 12

Characteristics of IP

Answer 12

IP was designed as a protocol with low overhead.
It provides only the functions that are necessary to deliver a packet from a source to a
destination over an interconnected system of networks.
The protocol was not designed to track and manage the flow of packets.
These functions, if required, are performed by other protocols at other layers, primarily TCP
at Layer 4.
These are the basic characteristics of IP:
* Connectionless - There is no connection with the destination established before
sending data packets.
* Best Effort - IP is inherently unreliable because packet delivery is not guaranteed.
* Media Independent - Operation is independent of the medium (i.e., copper, fiber
optic, or wireless) carrying the data.

Question 13

Connectionless

Answer 13

IP is connectionless, meaning that no dedicated end-to-end connection is created by IP
before data is sent.
Connectionless communication is conceptually similar to sending a letter to someone
without notifying the recipient in advance. The figure summarizes this key point.
Connectionless data communications work on the same principle.
As shown in the figure, IP requires no initial exchange of control information to establish an
end-to-end connection before packets are forwarded.

Question 14

Best Effort

Answer 14

IP also does not require additional fields in the header to maintain an established
connection.
This process greatly reduces the overhead of IP.
However, with no pre-established end-to-end connection, senders are unaware whether
destination devices are present and functional when sending packets, nor are they aware if
the destination receives the packet, or if the destination device is able to access and read
the packet.
The IP protocol does not guarantee that all packets that are delivered are, in fact,
received. The figure illustrates the unreliable or best-effort delivery characteristic of the
IP protocol.

Question 15

Media Independent

Answer 15

Unreliable means that IP does not have the capability to manage and recover from
undelivered or corrupt packets.
This is because while IP packets are sent with information about the location of delivery,
they do not contain information that can be processed to inform the sender whether
delivery was successful.
Packets may arrive at the destination corrupted, out of sequence, or not at all.
IP provides no capability for packet retransmissions if errors occur.
If out-of-order packets are delivered, or packets are missing, then applications using the
data, or upper layer services, must resolve these issues.
This allows IP to function very efficiently. In the TCP/IP protocol suite, reliability is the role of
the TCP protocol at the transport layer.
IP operates independently of the media that carry the data at lower layers of the protocol
stack.
As shown in the figure, IP packets can be communicated as electronic signals over copper
cable, as optical signals over fiber, or wirelessly as radio signals.

Question 16

How does the OSI data link layer contribute to the process of determining the maximum size of IP packets for transmission, and what impact does this have on network performance, particularly regarding fragmentation and latency?

Answer 16

The OSI data link layer is responsible for taking an IP packet and preparing it for
transmission over the communications medium.
This means that the delivery of IP packets is not limited to any particular medium.
There is, however, one major characteristic of the media that the network layer considers:
the maximum size of the PDU that each medium can transport.
This characteristic is referred to as the maximum transmission unit (MTU).
Part of the control communication between the data link layer and the network layer is the
establishment of a maximum size for the packet.
The data link layer passes the MTU value up to the network layer.
The network layer then determines how large packets can be.
In some cases, an intermediate device, usually a router, must split up an IPv4 packet when
forwarding it from one medium to another medium with a smaller MTU.
This process is called fragmenting the packet, or fragmentation.
Fragmentation causes latency.
IPv6 packets cannot be fragmented by the router.

Question 17

IPv4 Packet Header

Answer 17

IPv4 is one of the primary network layer communication protocols.
The IPv4 packet header is used to ensure that this packet is delivered to its next stop on
the way to its destination end device.
An IPv4 packet header consists of fields containing important information about the packet.
These fields contain binary numbers which are examined by the Layer 3 process.

Question 18

IPv4 Packet Header Fields

Answer 18

The binary values of each field identify various settings of the IP packet.
Protocol header diagrams, which are read left to right, and top down, provide a visual to
refer to when discussing protocol fields.
The IP protocol header diagram in the figure identifies the fields of an IPv4 packet.

Question 19

Significant fields in the IPv4 header include the following:

Answer 19

Version – Contains a 4-bit binary value set to 0100 that identifies this as na IPv4
packet.
* Differentiated Service sor DiffServ (DS) – Formerly called the type of service (ToS)
field, the DS field is na 8-bit field used to determine the priority of each Packet. The
six most significant bits of the DiffServ field are the differentiated services code
point (DSCP) bits and the last two bits are the explicit congestion notification (ECN)
bits.
* Time to Live (TTL) – TTL contains na 8-bit binary value that is used to limit the
lifetime of a Packet . The source device of the IPv4 packet sets the initial TTL value.
It is decreased by one each time the Packet is processed by a router. If the TTL
field decrements to zero, the router discards the Packet and sends na Internet
Control Message Protocol (ICMP) Time Exceeded message to the source IP
address. Because the router decrements the TTL of each Packet, the router must
also recalculate the Header Checksum.
Protocol – This field is used to identify the next level protocol. This 8-bit binary
value indicates the data payload type that the Packet is carrying, which enables the
network layer to pass the data to the appropriate upper-layer protocol. Common
values include ICMP (1), TCP (6), and UDP (17).
* Header Checksum – This is used to detect corruption in the IPv4 header.
* Source IPv4 Address – This contains a 32-bit binary value that represents the
source IPv4 address of the Packet. The source IPv4 address is always a unicast
address.
* Destination IPv4 Address – This contains a 32-bit binary value that represents the
destination IPv4 address of the Packet. The destination IPv4 is a unicast, multicast
or Broadcast address.

Question 20

What are the primary functions of the source and destination IP address fields in an IPv4 packet, and how do the Identification, Flags, and Fragment Offset fields facilitate the handling of packet fragmentation by routers?

Answer 20

The two most commonly referenced fields are the source and destination IP addresses.
These fields identify where the packet is coming from and where it is going.
Typically, these addresses do not change while travelling from the source to the
destination.
The Internet Header Length (IHL), Total Length, and Header Checksum fields are used to
identify and validate the packet.
Other fields are used to reorder a fragmented packet.
Specifically, the IPv4 packet uses Identification, Flags, and Fragment Offset fields to keep
track of the fragments.
A router may have to fragment an IPv4 packet when forwarding it from one medium to
another with a smaller MTU.
The Options and Padding fields are rarely used and are beyond the scope of this module.

Question 21

Limitations of IPv4

Answer 21

IPv4 is still in use today. This topic is about IPv6, which will eventually replace IPv4.
To better understand why you need to know the IPv6 protocol, it helps to know the
limitations of IPv4 and the advantages of IPv6.
Through the years, additional protocols and processes have been developed to address
new challenges. However, even with changes, IPv4 still has three major issues:
* IPv4 address depletion - IPv4 has a limited number of unique public addresses
available. Although there are approximately 4 billion IPv4 addresses, the increasing
number of new IP-enabled devices, always-on connections, and the potential
growth of less-developed regions have increased the need for more addresses.
* Lack of end-to-end connectivity - Network Address Translation (NAT) is a
technology commonly implemented within IPv4 networks. NAT provides a way for
multiple devices to share a single public IPv4 address. However, because the
public IPv4 address is shared, the IPv4 address of an internal network host is
hidden. This can be problematic for technologies that require end-to-end
connectivity.
* Increased network complexity – While NAT has extended the lifespan of IPv4 it
was only meant as a transition mechanism to IPv6. NAT in its various
implementation creates additional complexity in the network, creating latency and
making troubleshooting more difficult.

Question 22

IPv6 Overview

Answer 22

In the early 1990s, the Internet Engineering Task Force (IETF) grew concerned about the
issues with IPv4 and began to look for a replacement.
This activity led to the development of IP version 6 (IPv6).
IPv6 overcomes the limitations of IPv4 and is a powerful enhancement with features that
better suit current and foreseeable network demands.

Question 23

Improvements that IPv6 provides include the following:

Answer 23

Increased address space - IPv6 addresses are based on 128-bit hierarchical
addressing as opposed to IPv4 with 32 bits.
* Improved packet handling - The IPv6 header has been simplified with fewer
fields.
* Eliminates the need for NAT - With such a large number of public IPv6
addresses, NAT between a private IPv4 address and a public IPv4 is not needed.
This avoids some of the NAT-induced problems experienced by applications that
require end-to-end connectivity.
The 32-bit IPv4 address space provides approximately 4,294,967,296 unique addresses.
IPv6 address space provides 340,282,366,920,938,463,463,374,607,431,768,211,456, or
340 undecillion addresses.
This is roughly equivalent to every grain of sand on Earth.
The figure provides a visual to compare the IPv4 and IPv6 address space.

Question 24

IPv4 Packet Header Fields in the IPv6
Packet Header

Answer 24

One of the major design improvements of IPv6 over IPv4 is the simplified IPv6 header.
For example, the IPv4 header consists of a variable length header of 20 octets (up to 60
bytes if the Options field is used) and 12 basic header fields, not including the Options field
and Padding field.
For IPv6, some fields have remained the same, some fields have changed names and
positions, and some IPv4 fields are no longer required, as highlighted in the figure.
In contrast, the simplified IPv6 header shown the next figure consists of a fixed length
header of 40 octets (largely due to the length of the source and destination IPv6
addresses).
The IPv6 simplified header allows for more efficient processing of IPv6 headers.

Question 25

IPv6 Packet Header

Answer 25

The IP protocol header diagram in the figure identifies the fields of an IPv6 packet.
names and bit length of fields in an IPv6 header

Question 26

The fields in the IPv6 packet header include the following:

Answer 26

Version – This field contains a 4-bit binary value set to 0110 that identified this as
na IP version 6 packet.
* Traffic Class – This 8-bit field is equivalente to the IPv4 Differentiated Services (DS)
field.
* Flow Label – This 20-bit field suggests that all packets with the same flow label
receive the same type of handling by routers.
* Payload Length – This 16-bit field indicates the lenght of the data portion or payload
of the IPv6 packet. This does not include of the IPv6 header, which is fixed 40-byte
header.
* Next Header – This 8-bit field is equivalente to the IPv4 Protocol field. It indicates
the data payload type that the Packet is carrying, enabling the network layer to
pass the data to the appropriate upper-layer protocol.
* Hop Limit - This 8-bit field replaces the IPv4 TTL field. This value is decremented
by a valur of 1 by each router that forwards the Packet. When the counter reaches
0, the Packet is discarded, and an ICMPv6 Time Exceeded message is forwarded
to the sending host. This indicates that the Packet did not reaches its destination
because the hop limit was exceeded. Unlike IPv4, IPv6 does not include na IPv6
Header Checksum, because this function is performed at both the lower and upper
layers. This menas the checksum does not need to be recalculated by each router
when it decrements the Hop Limit field, which also improves network performance.
* Source IPv6 Address – This 128-bit field identifies the IPv6 address of the sending
host.
* Destination IPv6 Addess – This 128-bit field identifies the IPv6 address of the
receiving host.
An IPv6 packet may also contain extension headers (EH), which provide optional network
layer information.
Extension headers are optional and are placed between the IPv6 header and the payload.
EHs are used for fragmentation, security, to support mobility and more.
Unlike IPv4, routers do not fragment routed IPv6 packets.

Question 27

Host Forwarding Decision

Answer 27

With both IPv4 and IPv6, packets are always created at the source host.
The source host must be able to direct the packet to the destination host.
To do this, host end devices create their own routing table.
This topic discusses how end devices use routing tables.

Question 28

Another role of the network layer is to direct packets between hosts. A host can send a
packet to the following:

Answer 28

Itself - A host can ping itself by sending a packet to a special IPv4 address of
127.0.0.1 or an IPv6 address ::1, which is referred to as the loopback interface.
Pinging the loopback interface tests the TCP/IP protocol stack on the host.
* Local host - This is a destination host that is on the same local network as the
sending host. The source and destination hosts share the same network address.
* Remote host - This is a destination host on a remote network. The source and
destination hosts do not share the same network address.
The figure illustrates PC1 connecting to a local host on the same network, and to a remote
host located on another network.

Question 29

The method of determination varies by IP version:

Answer 29

In IPv4 - The source device uses its own subnet mask along with its own IPv4
address and the destination IPv4 address to make this determination.
* In IPv6 - The local router advertises the local network address (prefix) to all devices
on the network.
In a home or business network, you may have several wired and wireless devices
interconnected together using an intermediary device, such as a LAN switch or a wireless
access point (WAP).
This intermediary device provides interconnections between local hosts on the local
network.
Local hosts can reach each other and share information without the need for any additional
devices.
If a host is sending a packet to a device that is configured with the same IP network as the
host device, the packet is simply forwarded out of the host interface, through the
intermediary device, and to the destination device directly.
Of course, in most situations we want our devices to be able to connect beyond the local
network segment, such as out to other homes, businesses, and the internet.
Devices that are beyond the local network segment are known as remote hosts.
When a source device sends a packet to a remote destination device, then the help of
routers and routing is needed.
Routing is the process of identifying the best path to a destination.
The router connected to the local network segment is referred to as the default gateway.

Question 30

Default Gateway

Answer 30

The default gateway is the network device (i.e., router or Layer 3 switch) that can route
traffic to other networks.
If you use the analogy that a network is like a room, then the default gateway is like a
doorway.
If you want to get to another room or network you need to find the doorway.
On a network, a default gateway is usually a router with these features:
* It has a local IP address in the same address range as other hosts on the local
network.
* It can accept data into the local network and forward data out of the local network.
* It routes traffic to other networks.
A default gateway is required to send traffic outside of the local network.
Traffic cannot be forwarded outside the local network if there is no default gateway, the
default gateway address is not configured, or the default gateway is down.

Question 31

A Host Routes to the Default
Gateway

Answer 31

A host routing table will typically include a default gateway.
In IPv4, the host receives the IPv4 address of the default gateway either dynamically from
Dynamic Host Configuration Protocol (DHCP) or configured manually.
In IPv6, the router advertises the default gateway address or the host can be configured
manually.
In the figure, PC1 and PC2 are configured with the IPv4 address of 192.168.10.1 as the
default gateway.
Having a default gateway configured creates a default route in the routing table of the PC.
A default route is the route or pathway your computer will take when it tries to contact a
remote network.
Both PC1 and PC2 will have a default route to send all traffic destined to remote networks
to R1.

Question 32

Host Routing Tables

Answer 32

On a Windows host, the route print or netstat -r command can be used to display the host
routing table.
Both commands generate the same output.
The output may seem overwhelming at first, but is fairly simple to understand.
The figure displays a sample topology and the output generated by the netstat
r command.

Question 33

Entering the netstat -r command or the equivalent route print command displays three
sections related to the current TCP/IP network connections:

Answer 33

Interface List - Lists the Media Access Control (MAC) address and assigned
interface number of every network-capable interface on the host, including
Ethernet, Wi-Fi, and Bluetooth adapters.
* IPv4 Route Table - Lists all known IPv4 routes, including direct connections, local
network, and local default routes.
* IPv6 Route Table - Lists all known IPv6 routes, including direct connections, local
network, and local default routes.

Question 34

Router Packet Forwarding Decision

Answer 34

The previous topic discussed host routing tables. Most networks also contain routers, which
are intermediary devices. Routers also contain routing tables.
This topic covers router operations at the network layer.
When a host sends a packet to another host, it consults its routing table to determine where
to send the packet.
If the destination host is on a remote network, the packet is forwarded to the default
gateway, which is usually the local router.
What happens when a packet arrives on a router interface?
The router examines the destination IP address of the packet and searches its routing table
to determine where to forward the packet.
The routing table contains a list of all known network addresses (prefixes) and where to
forward the packet.
These entries are known as route entries or routes.
The router will forward the packet using the best (longest) matching route entry.

Question 35

IP Router Routing Table

Answer 35

The routing table of the router contains network route entries listing all the possible known
network destinations.

Question 36

The routing table stores three types of route entries:

Answer 36

Directly-connected networks - These network route entries are active router
interfaces. Routers add a directly connected route when an interface is configured
with an IP address and is activated. Each router interface is connected to a different
network segment. In the figure, the directly-connected networks in the R1 IPv4
routing table would be 192.168.10.0/24 and 209.165.200.224/30.
* Remote networks - These network route entries are connected to other routers.
Routers learn about remote networks either by being explicitly configured by an
administrator or by exchanging route information using a dynamic routing protocol.
In the figure, the remote network in the R1 IPv4 routing table would be 10.1.1.0/24.
* Default route – Like a host, most routers also include a default route entry, a
gateway of last resort. The default route is used when there is no better (longer)
match in the IP routing table. In the figure, the R1 IPv4 routing table would most
likely include a default route to forward all packets to router R2.

Question 37

A router can learn about remote networks in one of two ways:

Answer 37

Manually - Remote networks are manually entered into the route table using static
routes.
* Dynamically - Remote routes are automatically learned using a dynamic routing
protocol.

Question 38

Static Routing

Answer 38

Static routes are route entries that are manually configured.
The figure shows an example of a static route that was manually configured on router R1.
The static route includes the remote network address and the IP address of the next hop
router.
If there is a change in the network topology, the static route is not automatically updated
and must be manually reconfigured.
For example, in the figure R1 has a static route to reach the 10.1.1.0/24 network via R2.
If that path is no longer available, R1 would need to be reconfigured with a new static route
to the 10.1.1.0/24 network via R3.
Router R3 would therefore need to have a route entry in its routing table to send packets
destined for 10.1.1.0/24 to R2.

Question 39

Static routing has the following characteristics:

Answer 39

A static route must be configured manually.
* The administrator needs to reconfigure a static route if there is a change in the
topology and the static route is no longer viable.
* A static route is appropriate for a small network and when there are few or no
redundant links.

Question 40

Dynamic Routing

Answer 40

A dynamic routing protocol allows the routers to automatically learn about remote networks,
including a default route, from other routers.
Routers that use dynamic routing protocols automatically share routing information with
other routers and compensate for any topology changes without involving the network
administrator.
If there is a change in the network topology, routers share this information using the
dynamic routing protocol and automatically update their routing tables.
Dynamic routing protocols include OSPF and Enhanced Interior Gateway Routing Protocol
(EIGRP). The figure shows an example of routers R1 and R2 automatically sharing network
information using the routing protocol OSPF.

Question 41

Basic configuration only requires the network administrator to enable the directly connected
networks within the dynamic routing protocol. The dynamic routing protocol will
automatically do as follows:

Answer 41

Discover remote networks
* Maintain up-to-date routing information
* Choose the best path to destination networks
* Attempt to find a new best path if the current path is no longer available

Question 42

notes

Answer 42

When a router is manually configured with a static route or learns about a remote network
dynamically using a dynamic routing protocol, the remote network address and next hop
address are entered into the IP routing table.
As shown in the figure, if there is a change in the network topology, the routers will
automatically adjust and attempt to find a new best path.
Note: It is common for some routers to use a combination of both static routes and a
dynamic routing protocol.

Question 43

Introduction to an IPv4 Routing Table

Answer 43

Notice in the figure that R2 is connected to the internet. Therefore, the administrator
configured R1 with a default static route sending packets to R2 when there is no specific
entry in the routing table that matches the destination IP address. R1 and R2 are also
using OSPF routing to advertise directly connected networks.

Question 44

The show ip route privileged EXEC mode command is used to view the IPv4 routing table
on a Cisco IOS router.
The example shows the IPv4 routing table of router R1.
At the beginning of each routing table entry is a code that is used to identify the type of
route or how the route was learned. Common route sources (codes) include these:

Answer 44

L - Directly connected local interface IP address
* C – Directly connected network
* S – Static route was manually configured by an administrator
* O - OSPF
* D - EIGRP

Question 45

Network Layer Characteristics

Answer 45

The network layer (OSI Layer 3) provides services to allow end devices to exchange data
across networks.
IPv4 and IPv6 are the principle network layer communication protocols.
The network layer also includes the routing protocol OSPF and messaging protocols such
as ICMP.
Network layer protocols perform four basic operations: addressing end devices,
encapsulation, routing, and de-encapsulation.
IPv4 and IPv6 specify the packet structure and processing used to carry the data from one
host to another host.
IP encapsulates the transport layer segment by adding an IP header, which is used to
deliver the packet to the destination host.
The IP header is examined by Layer 3 devices (i.e., routers) as it travels across a network
to its destination.
The characteristics of IP are that it is connectionless, best effort, and media independent.
IP is connectionless, meaning that no dedicated end-to-end connection is created by IP
before data is sent.
The IP protocol does not guarantee that all packets that are delivered are, in fact,
received. This is the definition of the unreliable, or best effort characteristic.
IP operates independently of the media that carry the data at lower layers of the protocol
stack.

Question 46

IPv4 Packet

Answer 46

An IPv4 packet header consists of fields containing information about the packet.
These fields contain binary numbers which are examined by the Layer 3 process.
The binary values of each field identify various settings of the IP packet.
Significant fields in the IPv4 packet header include: version, DS, header checksum, TTL,
protocol, and the source and destination IPv4 addresses.

Question 47

IPv6 Packet

Answer 47

IPv6 is designed to overcome the limitations of IPv4 including: IPv4 address depletion, lack
of end-to-end connectivity, and increased network complexity.
IPv6 increases the available address space, improves packet handling, and eliminates the
need for NAT.
The fields in the IPv6 packet header include: version, traffic class, flow label, payload
length, next header, hop limit, and the source and destination IPv6 addresses.

Question 48

How a Host Routes

Answer 48

A host can send a packet to itself, another local host, and a remote host.
In IPv4, the source device uses its own subnet mask along with its own IPv4 address and
the destination IPv4 address to determine whether the destination host is on the same
network.
In IPv6, the local router advertises the local network address (prefix) to all devices on the
network, to make this determination.
The default gateway is the network device (i.e., router) that can route traffic to other
networks.
On a network, a default gateway is usually a router that has a local IP address in the same
address range as other hosts on the local network, can accept data into the local network
and forward data out of the local network, and route traffic to other networks.
A host routing table will typically include a default gateway.
In IPv4, the host receives the IPv4 address of the default gateway either dynamically via
DHCP or it is configured manually.
In IPv6, the router advertises the default gateway address, or the host can be configured
manually.
On a Windows host, the route print or netstat -r command can be used to display the host
routing table.

Question 49

Introduction to Routing

Answer 49

When a host sends a packet to another host, it consults its routing table to determine where
to send the packet.
If the destination host is on a remote network, the packet is forwarded to the default
gateway which is usually the local router.
What happens when a packet arrives on a router interface? The router examines the
packet’s destination IP address and searches its routing table to determine where to
forward the packet.
The routing table contains a list of all known network addresses (prefixes) and where to
forward the packet.
These entries are known as route entries or routes.
The router will forward the packet using the best (longest) matching route entry.
The routing table of a router stores three types of route entries: directly connected
networks, remote networks, and a default route.
Routers learn about remote networks manually, or dynamically using a dynamic routing
protocol.
Static routes are route entries that are manually configured.
Static routes include the remote network address and the IP address of the next hop
router.
OSPF and EIGRP are two dynamic routing protocols.
The show ip route privileged EXEC mode command is used to view the IPv4 routing table
on a Cisco IOS router.
At the beginning of an IPv4 routing table is a code that is used to identify the type of route
or how the route was learned. Common route sources (codes) include:
L - Directly connected local interface IP address
C - Directly connected network
S - Static route was manually configured by an administrator
O - Open Shortest Path First (OSPF)
D - Enhanced Interior Gateway Routing Protocol (EIGRP)