1. Chapter 5 OSI Network Layer
Modified by Profs. Chen and Cappellino

2. Objectives
The Network layer encapsulation allows the Transport layer contents to be passed to the destination within a network or on another network entirely.
Communication between networks is facilitated by a process called routing.
Upon completion of this chapter, you will be able to:
Identify the role of the Network layer as it describes communication from one end device to another end device.
Examine the most common Network layer protocol, Internet Protocol (IP), and its features for providing connectionless and best-effort service.
Understand the principles used to guide the division, or grouping, of devices into networks.
Understand the hierarchical addressing of devices and how this allows communication between networks.
Understand the fundamentals of routes, next-hop addresses, and packet forwarding to a destination network.

3. Network Layer – Communication from Host to Host
Transport layer (OSI Layer 4), which manages the data transport between the processes running on each end host VS
Network layer (OSI Layer 3) protocols ____ _______________ and ___________ used to ____________ from one host to another host.
Network layer doesn’t care what data it is actually caring
To accomplish this end-to-end transport, the Network layer uses 4 basic processes:
1. ______________________
2. ______________________
3. ______________________
4. ______________________

4. Communication from Host to Host
1. Addressing
Both end devices must have _____________________.
device AKA ___________________________
2. Encapsulation
Recall: Network layer PDU is call what?
As the data travels down the OSI from the source host, the Network layer (L3) receives the Transport layer (L4) PDU –AKA…
_______________________________________________
Among other info, the Packet contains two addresses:
The address of the host _______________________ known as the _____________________________________________
The address of the _________ host is called _______________.

5. Communication from Host to Host
3. Routing
Source and destination hosts are often not connected to the same network.
Network layer provides services to direct these packets to their destination host.
_____________________________________________, known as __________________, en route to a ___________________
Each router (network) that a packet passes through is called a _____.
The router ____________________________ and direct packets toward their destination.
Known as _____________________________
4. Decapsulation
Finally, the packet arrives at the destination host and is processed at the Network Layer.
The __________________ by the Network layer and __________ ________________________________________________

6. Network Layer Protocols
Protocols implemented at the Network layer that carry user data include:
Internet Protocol version 4 (___________)
Internet Protocol version 6 (____________)
Novell Internetwork Packet Exchange (______)
AppleTalk
The Internet Protocol (____ and IPv6) is the __________________________ data carrying protocol and will be the focus of this course.

7. IP V4 Protocol- most widely used…
Internet Protocol (IP) is a ____________________
IPv4 basic characteristics:
____________________ - No connection is established before sending data packets.
__________________________________ - No overhead is used to guarantee packet delivery.
_____________________________ - Operates independently of the medium carrying the data.
More to come on these…
FYI: IP version 6 (IPv6) is developed and being implemented in some areas.
IPv6 will operate alongside IPv4 and may replace it in the future.

8. IP V4 - Connectionless Service
In Connectionless data communications IP packets are __________________________ ____________________ that they are coming.
IP does ________________:
___________________________________ to establish an end-to-end connection,
________________________________ to maintain the connection.
With connectionless service, packets __________ at the destination _______________________.
upper layer services will have to resolve any issues.

9. IP V4 - Best Effort (unreliable)
Since protocols at other layers can manage reliability (for example, TCP of the TCP/IP protocol suite), IP is allowed to function very efficiently at the Network layer.
makes IP more _______________________________ for _________________________________.
IP packet header does not include fields required for reliable data delivery and so therefore is an unreliable protocol
Unreliable meaning IP does not have the capability to manage, and recover from, undelivered or corrupt packets.
____________________________ of packet delivery.
____________________________ for data.
_____________________________________________

10. IP V4 - Media Independent
Network layer is ___________________________ ________________________________
IP packet can be communicated electrically over cable, via light waves over fiber, or wirelessly as radio signals.
Data Link layer is concerned with transmission over the communications medium.
One major characteristic of the media that the _________________________________:
The _________________________________ ________________________________
Known as: Maximum Transmission Unit (_______).
The Data Link layer passes the MTU to the Network layer.
The Network layer then determines how large to create the packets.
A ____________________________________ when forwarding it from one media to another media if a smaller MTU is required.
This process is called ______________________ or fragmentation.

11. Encapsulating the Transport layer Segment
IPv4 encapsulates, the Transport layer segment
The process of encapsulating data by layer enables the _________ ____________________________ ___________________________.
_____________ - remains ______________________ during the Network layer processes.
Routers can implement different Network layer protocols concurrently
_________ only ___________ with the ________________________ that encapsulates the segment.

12. Red square indicates the HEADER portion of the packet
IPv4 Packet
Red square indicates the HEADER portion of the packet
We will discuss a few field- refer to the text or curriculum for details on all fields

13. IP V4 Packet Header fields
_____________________ (32 bits)
represents the source Network layer host address.
represents the destination Network layer host address.
______________________ (8 bits)
Indicates the ____________________________
The _____________________________________
Typically- initially set to 123 seconds
Value is ________________ at each hop
When the value becomes zero, the router discards the packet
Assures packets don’t loop around indefinitely between routers causing a routing loop.

14. Neat TTL tidbit…
Shows # of hops (TTL) for a packet of a particular type of OS running on a particular processor or platform

15. IP V4 Packet Header ________________ (8 bits)
Indicates the data __________________ that the packet is ___________________________.
Example values are: 01 ICMP; 06 TCP; 17 UDP
_____________________________ (8 bits)
The field is used to ___________________________.
This value enables router to give priority to certain traffic
Example: Telephony voice data might be considered high priority packets
_____________________________ (13 bits)
If a packet needs to be fragmented, the fragment offset field _______________________________________ ___________ in the reconstruction at the destination
Typically set to 0 indicating there is no offset
For additional field see curriculum- we have discussed the key ones…

16. Networks – Separating Hosts into Common Groups
As the number of hosts on the network grows, more planning is required to manage and address the network.
It is more practical and manageable to ________ ________________________________.
These smaller networks are often called __________ or ______________________.
Networks can be grouped based on three common factors that include:
1. _________________________________
Grouping hosts at the same location into separate networks can improve network management and operation.
Continued on next slide…

17. Separating Hosts into Common Groups cont…
2. ______________________________
Users who have ______________ typically use common software, tools, and access common hardware
We can often reduce the traffic by placing the ____________ to support the users in the ________________________________
Users using bandwidth intensive apps should be separated from other users
3. ________________________
______________________________________________ and devices based on the need for those resources
Ownership of a resource could be based on a company or part of a company or department

18. Why Separate Hosts into Networks (subnets)?
Improve Performance
Increase Security
Address Management issues
More to come on all of these…

19. Improve Performance
A ______________________________ __________________________ due to increased data traffic as well as increased control and management traffic
An increase in control traffic is often due to _________________________
A broadcast is a message _______________ ____________________________
Receiving host processing excess broadcast traffic will cause a degradation in it’s performance
____________________ generate ________ ______________________________
_________________________ to enable data communication on networks.
Broadcasts __________________, is known as a _____________________________
_____________________ also _______ ____________________________

20. Increase Security
The Internet has grown as businesses and organizations have developed their own IP networks that link to it.
Dividing networks based on ownership means that _________________________________ ___________________________________________________________________.
Same security concept applies within a corporate network
Dividing a network into subnets based on user access is a means ___________________________ ____________________________________________________________________________________
Security between networks is _____________ by devices such as a router or ____________ appliance

21. Address Management Issues
__________________________________________________________________
Dividing large networks so that hosts who need to communicate are grouped together allows host to know or easily learn the Layer two address of other hosts in that subnet
Increases efficiency
For ____________________________, the hosts only need to ______________ _________________, to which they send packets for all other destinations addresses.
This intermediary device is called a ________ and is generally a __________________________ __________________________________________

22. How do we Separate Hosts into Networks?
Hierarchical Addressing
More to come … 22

23. Hierarchical Addressing
Network layer addressing schemes are hierarchical.
Enables communication between networks and over the Internet.
What is an example of a hierarchical addressing scheme?
Using hierarchical addressing means that the __________ _____________________________________________________________________________
The _______________________ of the address indicates the ________________________________________________
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 network layer address of the destination host will have been used to deliver the packet.
____________________________ can be created if large networks need be ______________________________
Address would be: _____________________________________
More to come on next slide…

24. Hierarchical Addressing cont…
Logical 32-bit IPv4 address
Are _________________________________
Each octet is converted to its decimal value
4 decimal values separated by a dot (period).
For example
The IPv4 address is hierarchical and is made up of two parts (three parts when subnetted).
The ________ part identifies the ____________ and
the __________ part identifies a _________________.
In this example, the first three octets, ( ), can identify the network portion of the address, and the last octet, (57) identifies the host.
In a subnetted address:
first part is network, second is subnet, third is host
In subnetted network, the ____________________ _________________________________ of the address.
These borrowed host bits represent the different subnetworks within the range of the original network.

25. Hierarchical Addressing cont…
The number of bits of an address used as the network (and subnet) portion is called the _______________________
For example if a network uses 24 bits to express the network portion of an address the _______________ _____________________________-
In the devices in an IPv4 network, a separate 32-bit number called a _______________________ _________________________________-
________________ the prefix length or subnet mask enables the ________________________________.
Chapter 6 in this course will cover IPv4 network addressing and subnetworking in detail.

26. Support communication Outside our Network
_____________ a local area network or a _________________, _______________ _______________________________________________________ (router).
When a host needs to communicate with a network outside the local area network, the router connected to our local area network or subnet, acts as a gateway
The _________________________________ __________________________________
_______________ is the _________________ ___________________ that is connected to the same network as the host.
The router also needs a _________ that defines ______________________ next. This is called the ___________________

27. Carrying Data End to End via IP Packets
If communication is between hosts on two different networks
The local network delivers the packet from the _______________________________
The router ________________________ of the packet _________ address and forwards the packet to the appropriate interface.
If the destination network is (NOW) ________ ______________ to this router, the packet is _______________________________
If the destination network is _____________ ___________________, the packet is forwarded on to a second router- __________
The packet forwarding then becomes the responsibility of this second router.
This continues until the destination is reached or the max. number of hops is reached
At each hop, the forwarding decisions are based on the information in the IP packet header.
________________________________________ _____________________________________

28. A Gateway – The way out of Our Network
The gateway, also known as the _________________, is needed to _____________________________________
This gateway is a _________________ connected to the local network.
The gateway interface has a ______________________ that ______________________________________________
Default Gateway- configured on a host
Both the host IPv4 address and the gateway address must have the same network (and subnet, if used) portion of their respective addresses.
Confirming the Gateway
Host IP address, default gateway etc. can be viewed by issuing the ipconfig or _______________ command at the command line of a Windows computer.
The ______________ command used on a Linux or UNIX host

29. A Gateway – The way out of Our Network
A ___________________ ____________________________________________ for each packet that arrives at the gateway interface.
This forwarding process is referred to as ____________
If a route to a destination ____________________, the packet cannot be forwarded and is _________
Router uses a routing table to determine next hop…

30. A Route – The Path to a Network
The ___________ stores ________________ __________________________________
Connected networks are directly attached to one of the router interfaces.
These interfaces are the gateways for the hosts on different local networks.
____________________ can be __________ configured by the network admin. or _________ ____________________________________
Routes in a routing table have three main features:
___________________________
_________________________
_____________________________________ ______________________________________________________________________
The _____________________________ is used to forward a packet when the destination network is not represented by any other route in the routing table.
Note: The routing process and the role of metrics are the subject of a later course and will be covered in detail there.

31. Routing Table Entries- a closer look
A _________ in the routing table could refer to a _________________________________________________________________
When forwarding a packet, the ___________ __________________________________
EXAMPLE: As shown in the figure, if a packet arrives at a router with the destination address of , the router forwards the packet to a next-hop router associated with a route to network
If a route to is not listed on the routing, but a route to is available, the packet is forwarded to the next-hop router for that network.
Therefore, the precedence of route selection for the packet going to would be:
(Default route if configured)
5. Dropped
See next slide…

32. Default Route AKA __________________
A __________________ is a route that ______________________________
All or any is denoted by the address _______
The default route is used to _____________ for which there is _____________________ for the destination network.

33. A Route – Host Routing Table
______ also require a _______________ to ensure that Network layer packets created by that host, are directed to the correct destination network.
The host routing table is _________________ by the ____________________________________ ___________________________
Recall: the routing table in a _______ contains both _______________________
Local routing table info from a computer can be examined at the command line by issuing the _______________________ commands.
You can use the following options for the route command to modify the routing table contents:
route ADD
route DELETE
route CHANGE

34. The Next-Hop – Where the Packet Goes Next
A _____________________ of the device that will ____________________________
Next-hop for a host on the network, will be the ______ ______________________ (default gateway)
Next-hop for a router is the __________________ that will process the packet
In the routing table of a router, each route lists a next-hop for each destination address that is encompassed by the route.
__________________________________________________ because they are directly cabled to that router
Some _________________________________ indicating there are multiple paths to the same destination network.

35. Packet Forwarding – Moving toward its Destination
A router will do one of three things with a packet it receives:
____________________________________
______________________ if…
No default route is programmed OR
The destination network address doesn’t exist
Packet Examination
Packets that arrive at a router's interfaces are encapsulated as a Data Link layer (Layer 2) PDU.
What s the special name for the PDU at L2?
The ____________________________ so that the packet and destination IP address can be examined.
Next Hop Selection
Destination (IP) address in a packet is examined and the routing table is referenced
If the destination network is ______________ to the router, the packet is first _______________________ protocol then ______________________ to which that network is connected.
Otherwise, on to the next-hop router or dropped

36. Packet Forwarding – Using the Default Route
Review: As shown in the figure, 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.
This process may occur a number of times until the packet reaches its destination network.
Default routes are important:
It is not possible for all routers to have a route to every possible network on the Internet.
__________________________________________________________ at a router that has a specific route to the destination network.
Sees the default route (Gateway of last resort)

37. Routing Protocols – Sharing the Routes
Recall: route information within a routing table can be manually configured on the router or learned dynamically from other routers in the same internetwork.
For accurate routing decisions, the _____________ _______________________________________________________________ that the router can access.
Out-of-date routing information means that packets may not be forwarded to the most appropriate next-hop,
Two types of routing
________ Routing
_________ Routing
More to come on both…

38. Must be manually configured
Static Routing
______________________________________ to remote networks can be _________________ on the router.
This is known as static routing.
Upside- ________________________________
A _______________________________________________
If the router is connected to a number of other routers, knowledge of the internetworking structure is required for _________________________________
If the internetwork structure changes or if new networks become available, these changes ________________________________ ______________________________ that is running static routing
What do you think will happen if timely manual updating does not occur?
Must be manually configured

39. Dynamic Routing
The administrative maintenance involved and manual up-dating needed in Static routing makes it an undesirable routing option
Routing protocols are the answer:
Set of rules by which ___________________ ____________________________________
When routers become aware of changes to the network, the information is automatically passed on to other routers.
When a router _______________________ __________________________________________________________________ and, in turn, passes the information to other routers.
Common routing protocols are:
Routing Information Protocol (_________)
Enhanced Interior Gateway Routing Protocol (________________)
Open Shortest Path First (___________)

40. Dynamic Routing cont… Costs involved in routing protocols
First, the ________________________________ ___________________________ that consumes network bandwidth.
Second, _______________________________ by protocols such as EIGRP and OSPF- involves ________ ___________________________________
In many internetworks, a _______________ of static, dynamic, and default routes are used to provide the necessary routes.
Configuration of routing protocols covered extensively in later courses…