Cisco CCNA: Networks and Data Transfer

Unit 1. Communication between Layers

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1. Communication between Layers
       1.1 Data Transfer between Layers
       1.2 Network Communication
       1.3 Encapsulating Each Layer
       1.4 The Steps of Data Encapsulation
       1.5 Unit 1 Summary

Understanding how data transfer works in a networking environment requires thorough knowledge of the OSI reference model.
This course will refer to the OSI reference model to show you how information moves through a network. Communication between layers, data encapsulation, addressing, and routing are some of the topics which will be addressed.
In this unit, you use the OSI reference model to learn how systems in a network communicate from peer layer to peer layer, and how data is encapsulated as it moves between layers.

After completing this unit, you should be able to:

Discuss how peer layers communicate
Explain how data moves between layers of the OSI reference model
List the five steps of data encapsulation
Name the encapsulation data unit of each layer

This unit provides information that is relevant to the following CCNA exam objective:

Define and explain the five conversion steps of data encapsulation

Topic 1.1: Data Transfer between Layers

*Transmitting Data
When data is transmitted between systems in a network, each OSI layer on the sending system communicates indirectly with its peer layer on the receiving system. We call this peer-to-peer communication, with respect to the OSI reference model (not with respect to client-server networks).

*Protocol Data Units
In this peer-to-peer communication, layer of one system uses layer-specific PDUs (Protocol Data Units) to communicate with the peer layer of another system.
PDUs are units of information. They are layer-specific because each layer of the OSI model uses different protocols. As you learned earlier, protocols are the rules that govern how systems communicate with each other.

*Network Communications
In network communications, data doesn't move directly from one peer layer to another.
Instead, data moves down the OSI layers of the sending system across and up to the corresponding — or peer — layer of the receiving system.

*Data Direction
When data is transmitted and received across a network, each layer works with the layer directly above or below it, depending on the direction of data flow.

Question 1

Topic 1.2: Network Communication

*Data Transmission
In network communications, when data is transmitted, it moves down the layers until it reaches the Physical layer, where it is transmitted.

*Control Information
As data moves down the layers, each layer adds a header and maybe a trailer. These headers and trailers contain protocol-specific control information that allows peer layers to communicate. Altogether, the data, headers, and trailers make up the PDU.

*Encapsulation
This process of bundling data with protocol-specific headers and trailers is called encapsulation.
Another term for tunneling.

*Layer Dependence
As the data moves down each OSI layer, the header from the upper layers is treated as data.
Therefore, at any layer there is really only one header.

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Topic 1.3: Encapsulating Each Layer

*At the Application Layer
Now we'll go layer-by-layer through data encapsulation. Beginning at the Application level, user data is encapsulated before it moves down to the Presentation layer.

*At the Presentation Layer
When the Presentation layer receives the data, it encapsulates the data unit with its own protocol-specific header. The Presentation layer passes the data to the Session layer.

*At the Session Layer
The Session layer encapsulates the data with its header, then sends the data unit to the Transport layer.

*Defining a Message
A data unit at any of the top three layers of the OSI reference model — Application, Presentation, or Session — can be called a message.

*At the Transport Layer
Once data reaches the Transport layer, each successive layer encapsulates the data in a unique way to prepare it for transmission across the network.

*Transport Segments
The encapsulated data at the Transport Layer is called a segment. Each segment includes extra information that provides the message type, originating program, and protocols used. Segments provide for a reliable connection.

*At the Network Layer
The Network layer encapsulates the segment passed down by the Transport layer. At this layer the encapsulated data is called a packet or datagram. Packets information indicating the logical or network addresses of the source and destination system.

*At the Data Link Layer
The packets are encapsulated by the Data Link layer, creating a frame. Frames include information that is needed for the Data Link layer to complete its tasks. Such information includes the physical address of the source and destination system.

*At the Physical Layer
The encapsulated frame is now ready for transmission. The Physical layer transmits the data as bits over the physical network connection. A bit is a single digit that can be either a 0 or a 1 and is the smallest unit of data used by a computer.

*Receiving the Data
Once the data reaches its destination, the data is passed from the lower layers back to the upper layers. As the data is passed up, each header and trailer is de-encapsulated by the peer level on the receiving system.

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Topic 1.4: The Steps of Data Encapsulation

*Using Steps
You just learned how to characterize encapsulation according to OSI layers. You can also characterize encapsulation as a sequence of five steps.
Let's look at the details of each of these five steps.

*Step 1
The first step of data encapsulation begins with the conversion of user information to generic data for transmission across the network.
This process is referred to as building the data, and it takes place on the Application, Presentation, and Session layers.

*Step 2
Once the data has been prepared and a session has been established, the data is packaged for transport.
This packaging takes place at the Transport layer in the form of segments.

*Step 3
The segments continue their journey down the layers, and are assigned network addresses by the Network layer in the form of a packet or datagram.

*Step 4
Once the data receives network address information, it continues to the Data Link layer where it is given physical address information in the form of a frame.

*Step 5
The last step of data encapsulation is the conversion of the data into bits for transmission.

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* Exercise 1
Try working with data encapsulation.

**Examine the following table**
Step	Action
1	Sketch two OSI reference models. Label each layer and leave space between the two models.
2	Beginning with data, sketch the progress of encapsulation as data moves down the layers. Label each header appropriately and keep in mind that the data size will increase each time.
3	Label the encapsulation data unit of each layer, beginning with the Application layer.
4	Below the models, write the five steps of data encapsulation.
5	For more information on data encapsulation, visit Cisco's Web site, or perform an Internet search.

Topic 1.5: Unit 1 Summary

In this unit, you learned how each layer of the OSI reference model communicates with three other layers — its peer layer and the layers directly above and below it.
You analyzed how data is encapsulated as it moves down the layers of the sending system. After the data is transmitted across the network to the receiving system, you saw that the data is de-encapsulated layer by layer. In addition, you learned how to describe encapsulation as a sequence of five steps.
In the next unit, you'll learn about connection and connectionless sessions, and also controlling data flow.

Unit 2. Connection and Data Flow

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2. Connection and Data Flow
       2.1 Connection-Oriented vs. Connectionless
       2.2 Connection-Oriented Sessions
       2.3 Flow Control
              2.3.1 Buffering and Source Quench
              2.3.2 Windowing
       2.4 Expectational Acknowledgment System
       2.5 Unit 2 Summary

In this unit, you learn about the two broad categories of data transmission: connection-oriented and connectionless. You analyze how these two types of transmission compare and contrast.
Included in the discussion, is an examination of how flow control can promote reliable transmission in connection-oriented services.

After completing this unit, you should be able to:

Discuss the differences between a connection-oriented and connectionless session
Define an acknowledgment
List three types of flow control
Describe how each method of flow control works

This unit provides information that is relevant to the following CCNA exam objectives:

Describe connection-oriented network service and connectionless network service, and identify their key differences
Define flow control and describe the three basic methods used in networking

Topic 2.1: Connection-Oriented vs. Connectionless

*Connection-Oriented Services
Connection-oriented (CO) data transfer uses one logical path throughout a session of data transfer; also, the sending and receiving systems communicate with each other through Acknowledgments (ACKs). Acknowledgments are messages sent between networking systems to verify that some event has taken place. This event can be data transmission or synchronization of the two networking devices to get ready for data transfer.

*A Connection-Oriented Analogy
Connection-oriented service is like placing a phone call to a friend. You become the sending station, waiting for the call to be acknowledged by your friend before you broadcast your message.

*Connectionless Service
Connectionless service (CL), on the other hand, sends data on a connection that is permanently established, although the data does not necessarily follow one logical path. Also, connectionless service does not involve an exchange of acknowledgments (although sometimes the receiving system may send a confirmation that data was received).

*The Connectionless Analogy
Connectionless service is like leaving a message on an answering machine. You dial the number and leave a message. Does your friend get the message? You hope so, but since your friend isn't there to acknowledge your call, you can't be sure.

*Connection-Oriented Reliability
Connection-oriented services aren't guaranteed to be successful. But, because acknowledgments are exchanged, they are generally considered to be more reliable than connectionless services.
FTP is an example of a connection-oriented service.

*Connectionless for Delayed and Non-Sequential Transmission
Connectionless is better at allocating bandwidth and avoiding network failures. This is because CL data packets don't follow a static path for transmission, and the packets don't require sequential packet transmission.
This makes connectionless transmission useful for data that can experience delays and out-of-order sequencing. Connectionless services are good for voice and video transmission.

*CO and CL Protocols
TCP/IP networking uses both types of transmission on two different OSI layers. The protocol for connectionless services is IP, which is used at the Network layer. The protocol for the connection-oriented services is TCP, which is used at the Transport layer.

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Topic 2.2: Connection-Oriented Sessions

*Setting Up a CO Session
There are three steps to setting up a CO session:

Establishing the connection
Transferring data
Terminating the connection

Let's look at the details of each step.

*Step 1
To establish a connection-oriented session, both the transmitting and receiving devices send messages to verify that data transfer can take place. This is sometimes called session negotiation.
The timing of the sending and receiving devices is also set to a common time. This process is called synchronization.
The receiving system then sends an acknowledgment to confirm that both devices are ready to communicate.

*The Handshake
The messages sent between the sending and receiving devices to confirm synchronization are called handshakes.

*Three-Way Handshake
Some protocols, such as TCP on the Transport layer, use a "three-way handshake."
First, the sending system sends a request for synchronization. Then, the receiving system sends both a response that it is synchronized and an acknowledgment that it is synchronized and ready. The third and final part of the handshake occurs when the sending system sends an acknowledgment verifying that it is synchronized and the connection is established.

*Step 2
Once a connection-oriented session is established, data may be transported across the connection.
During transfer, both systems continually confirm that the data has been reliably transmitted and received.

*Step 3
When the transfer is complete, the session is terminated.

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Topic 2.3: Flow Control

*Flow Control
Flow control is a service available at the Transport layer with a connection-oriented session.
It is also available at the Data Link layer; however, we won't be discussing this type of flow control in this unit.

*Preventing Data Problems
Flow control is a method to prevent data loss or corruption by slowing or re-transmitting data flow if the data flow is about to flood its destination. Because it positively affects data flow, it directly reduces network congestion.

*Dropping Data
When data arrives at a destination more quickly than it can be processed or transferred, the destination device can drop data.

*Methods of Control
Data flow can be controlled through these methods:

Buffering
Using source-quench messages
Windowing

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Topic 2.3.1: Buffering and Source Quench

*The Role of a Buffer
A buffer is an allocation of memory that devices use to store transmitting data if it is arriving faster than it can be processed. When the buffer is full, incoming data is discarded.

*Sending Data Signals
To prevent data loss, a receiving device may send a message telling the transmitting device to stop sending data. When data can be accepted again, it sends a ready message.

*Slowing the Data Flow
Receiving devices use source-quench messages to tell a sending device that the receiver cannot keep up with the data flow. However, source-quench messages merely slow, rather than stop, data flow.

*Reducing Data Flow
For each data packet lost, the receiving device sends out a source-quench message. Sending systems, in turn, reduce data flow until the source-quench messages stop.

*Resuming Transmission Rates
When the sending system stops receiving source-quench messages, it gradually increases the rate of data transmission.

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Topic 2.3.2: Windowing

*Controlling Data Flow
The last type of flow control, Windowing, is a type of flow control that uses acknowledgments to control data flow.
After sending a pre-specified number of data packets, the sending device waits to receive an acknowledgment from the receiving device.
The receiving device only sends the acknowledgment when the pre-specified number of packets have been received. After the sending device receives the acknowledgment, it sends the next specified number of packets.

*The Cycle of Sending Data and Receiving ACKs
This cycle continues until all of the data is transmitted. Because this is an end-to-end method of flow control, the acknowledgments and data cycle back and forth without a problem.

*What Is a Window?
The number of packets which can be received before the acknowledgment is sent is called a window.

*Defining Window Size
For example, a window size of 3 means that the acknowledgment is sent after three packets have been received.
After the acknowledgment is received, the sending system sends the next set of three packets.

*The Problem with ACKs
When the receiving device sends an acknowledgment after receiving the specified number of packets, the sending device knows that no packets have been lost, corrupted, or duplicated.
Because the sending device must wait for the acknowledgments, throughput may be reduced. Throughput is the rate that data passes a specific point in a network.

*Making Adjustments
Windowing can maximize reliability and throughput by adjusting the number of packets which can be received without an acknowledgment.

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Topic 2.4: Expectational Acknowledgment System

*Lost or Damaged Data
While windowing helps control data flow by limiting the number of transmissions that can be sent without an acknowledgment, it does not totally address the problem of lost or damaged data.

*The Expectational Acknowledgment System
Many protocol systems, including TCP and SPX, use an expectational acknowledgment system to ensure data integrity.

*Maintaining Error Control
Expectational acknowledgment systems use positive acknowledgments to maintain error control.
The sending system sends a specified number of segments or packets and then stops, waiting for an acknowledgment from the receiving system.

*Timed Transmission
The sending system also sets a timer when the data transmission is complete. If the timer expires before an acknowledgment is received, the transmitting system re-sends the data.

*Asking to Retransmit
If a receiving system receives two out of three segments, it uses the positive acknowledgment to ask for retransmission.
The transmitting system then sends the missing segment or the entire data set, depending on the protocol.

Question 17

* Exercise 1
Try applying concepts of connection-oriented services, including flow control.

**Examine the following table**
Step	Action
1	Draw a sketch of the OSI reference model and label the Transport layer.
2	Explain the differences between connection-oriented and connectionless services. What is the degree of reliability with each? What are the advantages of each?
3	List the three methods of flow control.
4	Below each method of flow control, sketch a diagram showing how it helps to control data flow.
5	Explain how positive acknowledgments can be used.
6	Meet with a network administrator and discuss flow control. Ask the administrator about the forms of flow control used by the most common network protocols and the types of configurations that work best for different types of networks or hardware setups.

Topic 2.5: Unit 2 Summary

In this unit, you learned the difference between connection-oriented and connectionless services. You saw that flow control is only available with a connection-oriented service.
You studied the three methods of flow control — buffering, source-quench messages, and windowing. You also saw how positive acknowledgments are used in the expectational acknowledgment system.
In the next unit, you'll learn about the role the Data Link layer plays in data transfer.

This unit will revisit the Data Link layer of the OSI model. In a previous course, you learned that the Data Link layer handles physical addressing, error notification, and flow control. You also learned that the Data Link layer is subdivided into the Logical Link Control sublayer and the Media Access Control sublayer.
In this unit, you learn the tasks handled by each of these sublayers. You also learn the difference between physical and logical addressing, and the importance of each in reliable data transmission.

After completing this unit, you should be able to:

Name the Data Link sublayers
List the tasks associated with each sublayer
Define a physical address
Identify the components of a MAC address
Explain the process of address resolution

This unit provides information that is relevant to the following CCNA exam objective:

Describe data link and network addresses and identify key differences between them

Topic 3.1: The Data Link Sublayers

*Two Sublayers
The tasks associated with the Data Link layer are handled by two sublayers — Logical Link Control (LLC) and Media Access Control (MAC).

*Function Referral
The MAC focuses on lower-layer hardware functions, while the LLC focuses on higher-layer software functions.

Topic 3.1.1: Logical Link Control

*Handling Services
The LLC sublayer manages connection-oriented and connectionless services that function at the Data Link layer. These services include flow control and error checking during data transfer.

*Three Types of Services
There are three types of connectionless and connection-oriented transmission that LLC manages. They are:

Unacknowledged connectionless service
Acknowledged connectionless service
Connection-oriented service

*Unacknowledged CL
Unacknowledged connectionless service does not confirm data transfer, and the responsibility for reliable data transfer is often carried by the services on higher OSI layers.
Unacknowledged connectionless service can be compared to the person who calls a friend on the phone and leaves a message on their answering machine, without any confirmation that the friend will receive the message. It can also be compared to our postal service: you can send several letters to a friend, but the postal service will not confirm their arrival, or the order of arrival, at your friend's house.

*Acknowledged CL
Acknowledged connectionless transfer uses acknowledgements to confirm data transfer, but does not establish a logical connection for the transfer.
This can be compared to the person who sends registered letters to a friend. For registered letters, the postal service must send a receipt of delivery back to the sender.

*CO
Connection-oriented services use logical paths and acknowledgments to provide reliable data transfer.
This compares to the person who phones a friend and is able to establish a direct conversation.

*LLC Sublayer Specifications
The LLC sublayer follows the 802.2 protocol, which was set forth by the IEEE (Institute of Electrical and Electronic Engineers). The LLC works with several LAN and WAN protocols, although in this unit we discuss just LAN protocols and their relevance to the Data Link layer. Both LANs and WANs will be discussed in later courses.
The LLC sublayer is not governed by any specific LAN protocol. This is in contrast to the MAC sublayer. Because of the LLC, upper-layer protocols can function independently of LAN media access.

*Allowing Communications
The LLC sublayer allows several devices, and multiple upper level protocols, to share a single data connection. The LLC is also responsible for the flow control that functions in the Data Link layer.

*Defining Access Points
To allow the upper-layer protocols to share a single data connection, the LLC uses SAPs (Service Access Points). SAPs are two LLC fields that are included in Data Link frames.
The first field is the DSAP (Destination Service Access Point). The DSAP identifies the destination network node for the data packet. The second field is the SSAP (Source Service Access Point). The SSAP defines the source network node for the data packet.

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Topic 3.1.2: Media Access Control

*Allowing Access
While the LLC sublayer handles the final software stage of data transfer as it moves down the OSI reference model, the MAC sublayer allows access to the hardware medium.

*Addressing Services
The MAC sublayer also defines a physical address, which is a unique address that identifies each and every device connected to a LAN.
This address allows devices to recognize each other on the network.

*Address Terminology
Physical addresses are also called MAC addresses, Data Link addresses, Link layer addresses, and hardware addresses.
This course will usually refer to these physical addresses as MAC addresses.

*Defining a MAC Address
MAC addresses are comprised of a unique two-part number.
The number consists of 12 hexadecimal digits which are 48 bits in length.

*The OUI
The first part of a MAC address is the Organizational Unique Identifier (OUI). The OUI is six hexadecimal digits.
This number is assigned to each device manufacturer or vendor. The IEEE is responsible for assigning unique numbers.

*The Serial Number
The second part of a MAC address is also six hexadecimal digits.
It is assigned by the manufacturer and is typically the serial number of the interface.

*The NIC
The MAC address is burned into the Network Interface Card (NIC) that connects the device to the network.
For this reason, the MAC address is also sometimes referred to as a burned-in address (BIA).
MAC addresses are copied into RAM when the NIC initializes.

Question 21

* Exercise 1
Try working with the LLC and MAC sublayers.

**Examine the following table**
Step	Action
1	Sketch the OSI reference model. Label each layer.
2	At the Data Link layer, sketch the two sublayers and label them.
3	In each sublayer, list the tasks and specifications handled by that sublayer.
4	Below the model, sketch out an area and label it "MAC address."
5	Divide the MAC address into its component parts and label them.

Topic 3.2: Addressing

*Physical vs. Logical Addressing
To understand how MAC addresses work, it is important to understand the difference between physical and logical addressing.

*Physical Addresses
Physical addresses are unique for each network connection, and are resident in the hardware that connects a device to the network.

*Physical Addresses for Each Network
Most computer systems have a single physical address because they are connected to a single network, while devices such as routers may have several physical addresses — one for each network to which they are attached.

*Flat Address Space
Physical addresses use a flat address space. An example of a flat address space is your Social Security number, which is a unique number assigned just to you.

*Network Layer Addresses
By contrast, Network layer addresses are logical and hierarchical. An example of an hierarchical addressing system is the one used by the post office, which gives a person's name, street address, city, state, and zip. The envelope is routed through each level of the hierarchy until it reaches its destination.

*Network Address Functions
Network-layer addresses are also referred to as network addresses, logical addresses, virtual addresses, or protocol addresses. Generally, network addresses are used to move packets through internetworks, while hardware addresses are used to deliver packets between local devices, which are on the same network.
Protocols such as IP and IPX use network addressing. You will learn more about network addresses in the next unit.

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Topic 3.3: Resolving MAC Addresses

*Address Resolution
A device connected to a network can have at least two addresses — a physical address and a logical address. The process of resolving the differences between the two is called address resolution.

*Methods to Resolve Addresses
We'll discuss three ways to resolve addresses:

ARP
Hello protocol
Algorithms

*Address Resolution Protocol
ARP (Address Resolution Protocol) is one of the most common methods of address resolution. This protocol is used in TCP/IP.
For example, let's say device A and device B are on the same network. Device A broadcasts a message that says it is looking for the MAC address for device B.

*Returning a Local Address
All of the devices on the network receive and process the broadcast, but only device B will return its MAC address. Device A then stores the address in memory for the next time it needs to communicate with device B.

*Returning a Remote Address
In this example, device A and device B are on different networks connected by router C. As before, device A broadcasts a message that says it is looking for the MAC address for device B.

*Receiving the Broadcast
Router C, which serves both device A and device B, receives the broadcast. It recognizes that device B is located on another network. Therefore, router C sends its own (Router C's) MAC address to device A.

*For Future Reference
Device A then stores the MAC address for router C in memory for the next time it needs to communicate with device B.

*The Hello Protocol
Another method of address resolution uses the Hello protocol. The Hello protocol works by having each network device broadcast a Hello message.

*Broadcasting Hello Messages
Hello messages are also periodically broadcast to announce that a device is still operational. Devices can learn MAC addresses with the Hello messages.

*Algorithms for Predictability
Some protocols use algorithms to make addressing predictable, and therefore easier to resolve.
Xerox Network Systems (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV use algorithms.

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Question 24

* Exercise 2
Try working with address resolution protocols.

**Examine the following table**
Step	Action
1	Sketch a small network with at least two clients attached.
2	Use labeled arrows to indicate how address resolution occurs using ARP.
3	Sketch at least two small networks with at least two clients attached. Sketch a router connecting the networks.
4	Use labeled arrows to indicate how address resolution occurs with two clients on separate networks.
5	Use the small network sketch and show how the Hello protocol provides address resolution.
6	Below the sketches, list the protocols that use algorithms as a means of address resolution.

Topic 3.4: Unit 3 Summary

In this unit, you learned about the Data Link layer. You discovered that the tasks of the Data Link layer are handled by two sublayers — Logical Link Control and Media Access Control.
You also examined how physical addresses are assigned, as well as the difference between physical and logical addressing. In addition, you learned about different methods of address resolution.
In the next unit, you'll learn about network addressing and routing.

In this unit, you learn how packets of data are transferred between networks. You see how routers use network topology and routing tables to select the path that the data will travel.
In addition, you learn how network addressing is accomplished at the Network layer. You also see how addressing varies between protocols, and learn about TCP/IP addressing in particular.

After completing this unit, you should be able to:

Define path determination
Name the two parts of a network address
List the four common classes of the network part of a TCP/IP address
Identify the addressing schemes of three network protocols

This unit provides information that is relevant to the following CCNA exam objective:

List the key internetworking functions for the OSI Network layer

Topic 4.1: Routing Processes

*Routing
As you learned in a previous course, routers operate at the Network layer. Routing is the determination of a path for moving packets of information from a source to a destination.

*Routing Processes
Routing consists of two basic processes:

Path determination
Packet switching (on the Network layer, not the Data Link layer)

Topic 4.1.1: Path Determination

*Determining a Path
Path determination is the means by which all possible routes to a chosen destination are evaluated, and a single route is chosen.

*Evaluating Potential Paths
Path determination can be a very complex process. Routing protocols use information about the topology of the network to evaluate all potential paths to a given destination. Network topology information is either gathered by dynamic network processes, or configured by network administrators.

Topic 4.1.2: Packet Switching

*Moving Packets through the Network
Packet switching is the means by which packets are moved through the network along the chosen path. Packet switching differs from circuit switching in that it does not use a dedicated path for data flow.
Packet switching operates at two layers: the Network layer, which is what we are talking about here, and the Data Link layer, which switches packets using just the MAC address.

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Topic 4.2: Exchanging Path Information

*Router Operation
Routers operate at the Network layer of the OSI reference model and are vital components in the routing process.

*Unique Network Names
For routers to select a path for a packet, each network along a given path must have a unique name.

*Path Information
Each link in the network can be identified by a network address. When a router chooses a path, the address of each network crossed will be contained in the path information.

*Navigating between Networks
For example, in the image shown, the numbers assigned to the network connections are used by the routers as a network address. This information is used to navigate between networks until the packet reaches its destination.

*Consistent Addressing
Since network addresses play such an important role in path determination, consistent addressing across the entire network is vital. Consistent addressing helps to ensure accurate packet delivery and conserves resources by preventing unnecessary broadcasts by devices requesting addressing information.

Question 26

Topic 4.3: Defining Network Addresses

*Network and Host
Similar to MAC addresses, network addresses are typically composed of two parts. The first part of the address is the network portion, and the second is the host portion. For accurate packet delivery, both parts must be present.

*Identifying Network Details
The network portion of the address identifies the destination network for the packet. The host portion of the address identifies a specific device or port connected to the network.

*Network and Host Information
In the graphic shown, the network portion of the address shows the networks known to the router, while the host portion shows the devices connected to each network.

*The Host May Be a MAC
The information contained in the host portion of a packet may reflect the actual MAC address of the destination device.
This is true for LAN devices or systems using the IPX protocol.

*Successful Routing
However, unlike a MAC address which has a fixed relationship with the device, a network address has a logical relationship with the device. The network portion of the address is all that routing processes usually need to successfully route a packet. The host portion of the address is not required until the packet reaches the router connected to the destination network.

*Reaching the Destination
Once the packet has reached the home network of its destination device, the router then examines the host portion of the network address, and forwards the packet to the appropriate device.

*Multiple Network Addresses
For each network layer protocol that a device supports, it will have a separate network address. Therefore, if a device is connected to an IP and an IPX network, it will have a separate network address for each.

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Topic 4.4: Addressing Schemes

*Protocols and Addressing
Each networking protocol uses a slightly different addressing scheme for assigning network addresses. However, all of the protocols discussed in this course use the basic two-part network address formula.

Topic 4.4.1: TCP/IP

*TCP/IP Network Addresses
On TCP/IP networks, the IP protocol manages the Network-layer addresses. These addresses are 32 bits in length and are divided into two portions — a network portion and a host portion. The network portion is divided into classes by the Internet Request for Comments (RFC) 1117. The host portion is sometimes referred to as the node portion.
The four most common classes are A, B, C, and D. There is a Class E, but it is reserved for research.

*Class Specifics
Class A addresses use 8 bits for the network portion and 24 bits for the host portion.
Class B addresses use 16 bits for the network portion and 16 bits for the host portion.
Class C addresses use 24 bits for the network portion and 8 bits for the host portion.
Class D addresses are used for IP multicast addresses.

*IP Subdivisions
It is quite common to subdivide IP networks into smaller subnetworks. In these instances, the network portion for the subnetworks consists of:

network number, which is assigned by the InterNIC
subnetwork number, which is assigned by the local network administrator

*Host Address Assignment
For the host portion of the network address, IP requires the manual assignment of unique host addresses.
However, IPv6, which is version 6 of IP, will use MAC addresses. Incidentally, IPv6 is considered to be a replacement for IP 4, which is widely used today.

Topic 4.4.2: Other Protocols

*Addressing Schemes
Of course, routers can handle addressing schemes other than TCP/IP. The three most commonly used protocols other than TCP/IP are Novell IPX, AppleTalk, and X.25. We will show you the addresses for these on the next few pages, although we will cover each protocol and address in more detail in later courses.

*IPX Network Address
Novell IPX uses a hexadecimal number up to 32 bits in length for the network portion of a network address.
The host portion of the address is also a hexadecimal number and can be up to 48 bits in length.
The host portion is typically the MAC address of the destination device.

*AppleTalk Network Address
The network portion of an network address can be up to 16 bits in length, while the host portion can be up to 8 bits in length.
The host portion is usually assigned dynamically.

*X.25 Network Addresses
The network portion of an X.25 network address is a four-digit Data Network Identification Code (DNIC).
The DNIC is comprised of a two- or three-digit Data Country Code and a single network digit.
The host portion of the address can contain as many as 11 digits and is typically assigned by the WAN service provider.

Question 30

* Exercise 1
Try working with network addresses.

**Examine the following table**
Step	Action
1	List four major internetworking protocols.
2	Below each protocol sketch a representation of the network addressing scheme used by each. Sketch the network address, and divide it into the proper portions and sizes.
3	Does the protocol use MAC addresses for the host portion of the address? If so, indicate that with the word MAC in the host portion of the network address.
4	Which portion of the address is primarily used by routers to deliver a packet to the correct network? Indicate that with the word Router in the correct portion of the network address.
5	Which portion of the address is used to deliver a packet to the proper device or port? Indicate that with the word Local in the correct portion of the network address.

Topic 4.5: Unit 4 Summary

In this unit you analyzed how network addresses are used to deliver packets to the proper destination. You saw that network addresses can be divided into a network and a host portion. You also discovered that routers use only the network portion of a network address until the packet reaches the network of its destination. Once that happens, the host portion of the address is used.
Now that you have finished this course, you are equipped with much of the theory behind data transmission at the lower layers of the OSI reference model.

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Saturday, March 24, 2012