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SEMESTER 1 Chapter 5
OSI Network Layer
V 4.0
5.1.1.1 What services does the
network layer provide?
What are the 4 basic
processes used at the
network layer?
What is the PDU of the
network layer?
What information is
contained in the layer 3
PDU?
What is the role of a router?
What is it called when a
packet passed through an
intermediary device?
What happens to the
transport layer PDU as the
packet moves through the
network?
What does the network
layer protocol specify?
5.1.1.2 List the 5 Network layer
protocols.
Which protocol will be the
focus of this class?
5.1.2.1 What are the basic
characteristics of IPv4?
5.1.3
Describe what a
connectionless service in
networking means?
What is the main issue that
may occur in a
services to exchange the individual pieces of
data over the network between identified end
devices.
Addressing
Encapsulation
Routing
Decapsulation
Packet
Source and Destination IP address
to select paths for and direct packets toward
their destination. This process is known as
routing
Hop
It stays the same
the packet structure and processing used to
carry the data from one host to another host
Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service
(CLNS/DECNet)
IP
Connectionless - No connection is established
before sending data packets.
Best Effort (unreliable) - No overhead is used
to guarantee packet delivery.
Media Independent - Operates independently
of the medium carrying the data.
requires no initial exchange of control
information to establish an end-to-end
connection before packets are forwarded, nor
does it require additional fields in the PDU
header to maintain this connection
Connectionless packet delivery may, however,
result in packets arriving at the destination out
connectionless exchange?
5.1.4
5.1.5
5.1.6
What does unreliable mean
in networking?
Which layer is responsible
for adding reliability to
packet delivery?
Describe media
independent.
What layer is responsible
for preparing a packet for
the media?
What is the one
characteristic of the network
layer that is dependent on
the media?
What portion of the IP
packet does a router use to
make forwarding decisions?
What remains untouched as
packets travel through the
network?
5.1.7.1 What are the 6 key fields in
an IP packet?
What happens to a packet if
the TTL field reaches 0?
5.1.7.2 What is the checksum field
used for?
5.2.1
What are three methods of
determining how to divide
your network?
5.2.1.2 Why would you group users
geographically?
Why would you group users
by task?
Why would you group
networks by ownership?
5.2.2
What are the 3 main issues
as networks grow larger?
of sequence. If out-of-order or missing packets
create problems for the application using the
data, then upper layer services will have to
resolve these issues.
Unreliable means simply that IP does not have
the capability to manage, and recover from,
undelivered or corrupt packets.
Transport Layer
IPv4 and IPv6 operate independently of the
media that carry the data at lower layers of the
protocol stack.
Physical Layer
Maximum Transmission Unit MTU
Header
In all cases, the data portion of the packet that is, the encapsulated Transport layer PDU remains unchanged during the Network layer
processes.
IP Source Address
IP Destination Address
Time-to-Live (TTL)
Type-of-Service (ToS)
Protocol
Fragment Offset
It is discarded
To determine if the header has reached the
destination intact.
Geographic location
Purpose
Ownership
To improve manageability and network
performance.
To improve performance based on network
requirements of each task.
To maintain control and limit access.
Performance degradation
Security issues
Address Management
What is a network
broadcast?
What is broadcast domain?
5.2.3
5.2.4
5.2.5
Is it good or bad to increase
the number of broadcast
domains in your network?
What is the major
advantage of dividing
networks by ownership?
What efficiency does
dividing large networks into
separate networks provide
to the hosts?
What is the device called
that hosts send information
to that must travel outside
their network?
What is hierarchical
addressing?
What is the example
presented that describes
hierarchical addressing in
our daily lives?
5.2.5
How many bits are in an
IPv4 address?
How is the ip address
divided?
What portion of the IP
address does a router use?
5.3.1
What is the actual address
used for a default gateway?
What is the route that the
router forwards a packet to
called?
5.3.2
What decisions do routers
make as a packet enters an
interface?
5.3.3
How is the default gateway
configured on a host
running windows?
What portion of the IP
address must be the same
for the host and gateway?
5.3.3.2 What command is issued at
the command prompt of
windows to see the default
A broadcast is a message sent from one host
to all other hosts on the network.
The network area in which all hosts receive
broadcasts
Good
access to and from resources outside each
network can be prohibited, allowed, or
monitored
hosts who need to communicate are grouped
together reduces the unnecessary overhead of
all hosts needing to know all addresses.
Gateway
A hierarchical address uniquely identifies each
host. It also has levels that assist in forwarding
packets across internetworks, which enables a
network to be divided based on those levels.
Zip codes and house addresses
32
IPv4 addresses are divided in four groups of
eight bits (octets)
Network
The router interface IP address at the edge of
the network.
Next-hop address
Is the network directly connected or not. Yes
forward to that network. No forward to the
next-hop router.
On a Windows computer, the Internet Protocol
(TCP/IP) Properties tools are used to enter the
default gateway IPv4 address
Network and Subnetwork
ipconfig
gateway setting?
5.3.3.3 What is it called when a
router makes forwarding
decisions?
What is the map called that
a router uses to make
forwarding decisions (Hint:
look at the figure)?
5.3.4
What does a routing table
contain?
What are the 3 main
features of routes in a
routing table?
What happens if the packet
does not have an address
contained in the routing
table?
What is a default route used
for?
5.3.4.2 What is the command to
view the routing table for a
host?
What is the destination
network IP address and
subnet mask for the default
network interface?
What three commands are
listed that allow you to
change the routing table?
5.3.5
What does the address in
the routing table represent?
Routing
Routing Table
The routing table stores information about
connected and remote networks.
Destination network
Next-hop
Metric
The packet is dropped
The default route is used when the destination
network is not represented by any other route
in the routing table.
Netstat
0.0.0.0 0.0.0.0
route ADD
route DELETE
route CHANGE
represents a range of host addresses and
sometimes a range of network and host
addresses
If more than one route
The route that is most specific for that IP
exists in the routing table for address
an address, which one will
be selected?
5.3.5.2 When is the default route
When the IP address does not match any
used?
other entry in the routing table
5.3.6
What is the next-hop?
the address of the device that will process the
packet next
Why does a directly
There are no intermediary devices and the
connected device not have
packet is directly forwarded
a next-hop?
5.3.7.1 What are the three
Forward it to the next-hop router
decisions a router can make Forward it to the destination host
about a packet?
Drop it
Which layer does a router
3
process packets at?
What layer is the packet
encapsulated at before it
leaves the router?
5.3.7.2 What is another name for a
default gateway?
Why would a packet take a
different route than another
packet that is part of the
same data stream?
Why are default routes so
important to routers?
5.3.7.3 What happens if a router
has not route for the packet
and does not have a default
route?
Why doesn’t IP have error
notification?
5.4.1
What does a router need to
forward a packet?
What happens if routing
table are not up to date?
What are two ways a router
can learn about routes?
5.4.2
5.4.3
What is a static route?
What must be done if the
internetwork changes?
What is the definition of a
dynamic routing protocol?
What happens when a
router receives information
about changes in routes?
What are 3 common routing
protocols?
What are the 2 costs listed
with using a dynamic
protocol?
What advantages are listed
for static routing?
Data Link Layer 2
Gateway of last resort
Rotuers may learn a new path between
packets
because the gateway router is not likely to
have a route to every possible network on the
Internet.
The packet is dropped
It would reduce efficiency and add overhead
Next-hop
that packets may not be forwarded to the most
appropriate next-hop, causing delays or packet
loss
route information can be manually configured
on the router or learned dynamically from other
routers in the same internetwork
A manually configured route
Static routes must be reconfigured
set of rules by which routers dynamically share
their routing information
It updates its’ routing table
Routing Information Protocol (RIP)
Enhanced Interior Gateway Protocol (EIGRP)
Open Shortest Path First (OSPF)
Overhead that consumes network bandwidth
Processing capacity necessary
No network overhead produced
No processing capacity used
Administrative distance is configured to ensure
efficient and effective routing
* Networks can be grouped based on factors
– Geographic location
– Purpose
– Ownership
* Common issues with large networks are
– Performance degradation
– Security issues
– Address Management
* You should still be expected to know what is broadcast domain and know how to count
them?
* The role of the Network layer is to transfer data from the host that originates the data
to the host that uses it.
– If the destination host is in the same network,
– the packet is delivered between the two hosts on the local media without
the need for a router.
– If communication is between hosts in different networks,
– The local network delivers the packet from the source to its gateway
router.
– The router examines the network portion of the packet destination
address and forwards the packet to the appropriate interface.
– If the destination network is (NOW) directly connected to this router, the
packet is forwarded directly to that host.
– If the destination network is not directly connected, the packet is
forwarded on to a second router.
* What information is added during the encapsulation of the OSI network layer?
– When referring to the Network layer, we call this PDU a packet.
– The address of the host to which it is being sent. This address is referred to as
the destination address.
– The address of the originating host is called the source address.
*IPv4 basic characteristics:
– Connectionless - No connection is established before sending data packets.
– Best Effort (unreliable) - No overhead is used to guarantee packet delivery.
– Media Independent - Operates independently of the medium carrying the data.
* Time-to-Live
– The Time-to-Live (TTL) indicates the remaining "life" of the packet.
– The TTL value is decreased by at least one each time the packet is processed by
a router (that is, each hop).
– When the value becomes zero, the router discards or drops the packet and it is
removed from the network data flow.
– This mechanism prevents packets that cannot reach their destination from being
forwarded indefinitely between routers in a routing loop.
– Decrementing the TTL value at each hop ensures that it eventually becomes
zero and that the packet with the expired TTL field will be dropped.
* To support data communications between networks over internetworks, Network layer
addressing schemes are hierarchical.
 Layer 3 addresses supply the network portion of the address. Routers
forward packets between networks by referring only to the part of the
Network layer address that is required to direct the packet toward the
destination network.
 By the time the packet arrives at the destination host network, the whole
destination address of the host will have been used to deliver the packet.
* What is the purpose of “default gateway” address configured on the PC? What
address is used to configure the “default gateway” address.
 Keep in mind that it is not feasible for a particular host to know the address of
every device on the Internet with which it may have to communicate.
– To communicate with a device on another network, a host uses the
address of this gateway, or default gateway, to forward a packet outside
the local network.
– This gateway address is the address of a router interface that is
connected to the same network as the host.
* As each packet arrives at a router, the destination network address is examined
and compared to the routes in the routing table. What are the ways that you can
configure the “routing table”?
 Routes to remote networks with the associated next hops can be manually
configured on the router.
o This is known as static routing.
 Routing protocols are the set of rules by which routers dynamically share their
routing information.
o As routers become aware of changes to the networks for which they
act as the gateway, or changes to links between routers, this
information is passed on to other routers.
o the exchange of route information adds overhead that consumes
network bandwidth
o provide routers with up-to-date routing tables
* What is the purpose of “default route” in a router?
 if the routing table does not contain a more specific route entry for an arriving
packet, the packet is forwarded to the interface indicated by a default route, if
one exists.
– The default route is also known as the Gateway of Last Resort.
– This process may occur a number of times until the packet reaches its
destination network.
– If a route to a destination network does not exist, the packet cannot be
forwarded.
 [Tony]: Dropped the packet
* Although, we only cover briefly about the ‘routing table”. You are expected to know
what does it looks like and how to read the router table and know what information are
included in the routing table?
 a routing table have three main features:
– Destination network
– Metric
–
Next-hop
 A next-hop is the address of the device that will process the packet
next.
 For a host on a network, the address of the default gateway (router
interface) is the next-hop for all packets destined for another
network.
 In the routing table of a router, each route lists a next hop for each
destination address that is encompassed by the route.
* Which intermediate devices can be configured security and protect the network?
–
Security between networks is implemented in an intermediary device (a
router or firewall appliance) at the perimeter of the network.
* You should know how to read the output shown below. Based on the output shown
below, which address is the PC IP address and which address is the default gateway
address? What is your subnet mask?