At this point, you know about protocols and why they are important for network communication. To keep the analogy of sending a card to your relative going, think about the card, and the envelope, and your relative‛s address, as well as your own address. All of these parts are required to correctly deliver the card from your house to your relative‛s house. This chapter helps you to understand the different types of network addresses and the different parts of a network address. You will use this knowledge every day of your career, so don‛t waste any time.
Ethernet is a protocol used to deliver information from one Ethernet network interface card (NIC) to another Ethernet NIC on the same network. This section explains the process known as encapsulation and how the fields of an Ethernet frame are used to transmit the embedded information.
When sending a letter, a letter writer uses an accepted format to ensure that the letter is delivered and understood by the recipient. In the same way, a message that is sent over a computer network follows specific format rules for it to be delivered and processed.
The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope. Just as a letter is encapsulated in an envelope for delivery, computer messages are encapsulated.
Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host. The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated. Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.
A common example of requiring the correct format in human communication is when sending a letter, as shown in Figure 6-1. An envelope has the address of the sender and receiver, each located at the proper place. If the destination address and formatting are not correct, the letter is not delivered.
Figure 6-1 Format for Sending a Letter
Similar to sending a letter, a message that is sent over a computer network follows specific format rules for it to be delivered and processed.
Internet Protocol (IP) is a protocol with a similar function to the envelope example. In Figure 6-2, the fields of the Internet Protocol version 6 (IPv6) packet identify the source of the packet and its destination. IP is responsible for sending a message from the message source to a destination over one or more networks.
When you are connecting to a network using a wired interface, you are using the Ethernet protocol. Even most wireless networks ultimately connect to a wired Ethernet network. Ethernet is an important data link layer protocol used in LANs and most wide-area networks (WANs).
In the early days of networking, each vendor used its own proprietary methods of interconnecting network devices and networking protocols. If you bought equipment from different vendors, there was no guarantee that the equipment would work together. Equipment from one vendor might not communicate with equipment from another.
As networks became more widespread, standards were developed that defined rules by which network equipment from different vendors operated. Standards are beneficial to networking in many ways:
Facilitate design
Simplify product development
Promote competition
Provide consistent interconnections
Facilitate training
Provide more vendor choices for customers
There is no official local-area networking standard protocol, but over time, one technology, Ethernet, has become more common than the others. Ethernet protocols define how data is formatted and how it is transmitted over the wired network. The Ethernet standards specify protocols that operate at Layer 1 and Layer 2 of the OSI model. Ethernet has become the de facto standard, which means that it is the technology used by almost all wired local-area networks, as shown in Figure 5-8.
Figure 5-8 The Evolution from Proprietary LAN Protocols to Ethernet
Ethernet Evolution (5.4.2)
The Institute of Electrical and Electronic Engineers, or IEEE (pronounced “eye-triple-e”), maintains the networking standards, including Ethernet and wireless standards. IEEE committees are responsible for approving and maintaining the standards for connections, media requirements, and communication protocols. Each technology standard is assigned a number that refers to the committee that is responsible for approving and maintaining the standard. The committee responsible for Ethernet standards is 802.3.
Since the creation of Ethernet in 1973, standards have evolved for specifying faster and more flexible versions of the technology. This ability for Ethernet to improve over time is one of the main reasons that it has become so popular. Each version of Ethernet has an associated standard. For example, 802.3 100BASE-T represents the 100 megabit Ethernet using twisted-pair cable standards. The standard notation translates as follows:
100 is the speed in Mbps.
BASE stands for baseband transmission.
T stands for the type of cable—in this case, twisted-pair.
Early versions of Ethernet were relatively slow at 10 Mbps. The latest versions of Ethernet operate at 10 gigabits per second and more. Imagine how much faster these new versions are than the original Ethernet networks.
All communication requires a way to identify the source and destination. The source and destination in human communication are represented by names.
When your name is called, you listen to the message and respond. Other people in the room may hear the message, but they ignore it because it is not addressed to them.
On Ethernet networks, a similar method exists for identifying source and destination hosts. Each host connected to an Ethernet network is assigned a physical address that serves to identify the host on the network.
Every Ethernet network interface has a physical address assigned to it when it is manufactured. This address is known as the Media Access Control (MAC) address. The MAC address identifies each source and destination host on the network, as shown in Figure 5-9.
Figure 5-9 MAC Addresses Identify Unique Hosts on a LAN
Lab—Determine the MAC Address of a Host (5.4.5)
In this lab, you will complete the following objectives:
Determine the MAC address of a Windows computer on an Ethernet network using the ipconfig /all command.
Analyze a MAC address to determine the manufacturer.
The following is a summary of each topic in the chapter:
The Rules—All communication methods have three elements in common. The first is the message source, or sender. Message sources are people or electronic devices that need to communicate a message to other individuals or devices. The second is the destination, or receiver, of the message. The destination receives the message and interprets it. The third is the transmission medium, or channel. It provides the pathway over which the message can travel from source to destination.
Among the protocols that govern successful human communication are an identified sender and receiver, an agreed-upon method of communicating, common language and grammar, speed and timing of delivery, and confirmation or acknowledgment requirements. Networking protocols define the message format, message size, timing, encoding, and message patterns over the local network.
Communication Standards—Networking and Internet standards ensure that all devices connecting to the network implement the same set of rules or protocols in the same manner. Using standards, different types of devices are able to send information to each other over the Internet. These standards are developed, published, and maintained by a variety of organizations. When a new standard is proposed, each stage of the development and approval process is recorded in a numbered RFC document so that the evolution of the standard is tracked. RFCs for Internet standards are published and managed by the IETF.
Network Communication Models—A stack illustrates the protocols as a layered hierarchy, with each higher-level protocol depending on the services of the protocols shown in the lower levels. The separation of functions enables each layer in the stack to operate independently of others.
The layered model has many benefits:
Assists in protocol design, because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below
Fosters competition because products from different vendors can work together
Enables technology changes to occur at one level without affecting the other levels
Provides a common language to describe networking functions and capabilities
The suite of TCP/IP protocols used for Internet communications follows the structure of the stack model. The two basic types of models to describe the functions that must occur for network communications to be successful are protocol models and reference models. The most widely known internetwork reference model is the OSI model. The OSI model breaks down network communications into multiple processes. Each process is a small part of the larger task.
The protocols that make up the TCP/IP protocol suite can be described in terms of the OSI reference model. The functions that occur at the Internet layer in the TCP/IP model are contained in the network layer of the OSI model. The transport layer functionality is the same between both models. However, the network access layer and the application layer of the TCP/IP model are further divided in the OSI model to describe discrete functions that must occur at these layers.
Ethernet—There is no official LAN standard protocol, but over time, Ethernet has become more common than the others. Ethernet protocols define how data is formatted and how it is transmitted over the wired network. The Ethernet standards specify protocols that operate at Layer 1 and Layer 2 of the OSI model. Ethernet standards have evolved for specifying faster and more flexible versions of the technology. Each version of Ethernet has an associated standard. Each host connected to an Ethernet network is assigned a physical address that serves to identify the host on the network. Every Ethernet network interface has a physical address assigned to it when it is manufactured. This address is known as the MAC address. The MAC address identifies each source and destination host on the network.
The application layer contains protocols used for process-to-process communications.
6–Presentation
The presentation layer provides for common representation of the data transferred between application layer services.
5–Session
The session layer provides services to the presentation layer to organize its dialogue and to manage data exchange.
4–Transport
The transport layer defines services to segment, transfer, and reassemble the data for individual communications between the end devices.
3–Network
The network layer provides services to exchange the individual pieces of data over the network between identified end devices.
2–Data Link
The data link layer protocols describe methods for exchanging data frames between devices over a common media.
1–Physical
The physical layer protocols describe the mechanical, electrical, functional, and procedural means to activate, maintain, and de-activate physical connections for bit transmission to and from a network device.
Upper and Lower Layers of the OSI Model (5.3.6)
You can visualize how data moves across a network by using the seven layers of the OSI model, as shown in Table 5-3. The OSI model breaks down network communication into multiple processes, as shown in Table 5-4. Each process is a small part of the larger task.
Table 5-4 Common Components of the Layers of the OSI Model
Group
Layer Number
Layer Name
Common Network Components Associated with This Layer
Upper Layers
7
Application
Network-aware applications
6
Presentation
Email
5
Session
Web browsers and servers
File transfer
Name resolution
Lower Layers
4
Transport
Video and voice streaming mechanisms
Firewall filtering lists
3
Network
IP addressing
Routing
2
Data Link
Network interface cards and drivers
Network switching
WAN connectivity
1
Physical
Physical medium (copper twisted-pair, fiber-optic cables, wireless transmitters)
For example, in a vehicle manufacturing plant, the entire vehicle is not assembled by one person. Rather, the vehicle moves from station to station where specialized teams add specific components. The complex task of assembling a vehicle is made easier by breaking it into manageable and logical tasks. This process also makes troubleshooting easier. When a problem occurs in the manufacturing process, it is possible to isolate the problem to the specific task where the defect was introduced and then fix that problem.
In a similar manner, the OSI model helps you troubleshoot by focusing on a specific layer to identify and resolve network problems. Networking teams often refer to different functions occurring on a network by the number of the OSI model layer that specifies that functionality. For example, the process of encoding the data bits for transmission across the media occurs at Layer 1, the physical layer. The formatting of data so it can be interpreted by the network connection in your laptop or phone is described at Layer 2, the data link layer.
OSI Model and TCP/IP Model Comparison (5.3.7)
Because TCP/IP is the protocol suite in use for Internet communications, why do you need to learn the OSI model as well? The TCP/IP model is a method of visualizing the interactions of the various protocols that make up the TCP/IP protocol suite. It does not describe general functions that are necessary for all networking communications. It describes the networking functions specific to those protocols in use in the TCP/IP protocol suite. For example, at the network access layer, the TCP/IP protocol suite does not specify which protocols to use when transmitting over a physical medium, nor the method of encoding the signals for transmission. OSI Layers 1 and 2 discuss the necessary procedures to access the media and the physical means to send data over a network.
The protocols that make up the TCP/IP protocol suite can be described in terms of the OSI reference model. The functions that occur at the Internet layer in the TCP/IP model are contained in the network layer of the OSI model, as shown in Figure 5-7. The transport layer functionality is the same between both models. However, the network access layer and the application layer of the TCP/IP model are further divided in the OSI model to describe discrete functions that must occur at these layers.
Figure 5-7 The OSI and TCP/IP Models
The key similarities are in the transport and network layers; however, the two models differ in how they relate to the layers above and below each layer:
OSI Layer 3, the network layer, maps directly to the TCP/IP Internet layer. This layer is used to describe protocols that address and route messages through an internetwork.
OSI Layer 4, the transport layer, maps directly to the TCP/IP transport layer. This layer describes general services and functions that provide ordered and reliable delivery of data between source and destination hosts.
The TCP/IP application layer includes several protocols that provide specific functionality to a variety of end-user applications. The OSI model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce applications that operate on networks.
Both the TCP/IP and OSI models are commonly used when referring to protocols at various layers. Because the OSI model separates the data link layer from the physical layer, it is commonly used when referring to these lower layers.
Network communication models help you understand the various components and protocols used in network communications. These models help you see the function of each protocol and their relationship to other protocols.
Successful communication between hosts requires interaction between many protocols. These protocols are implemented in software and hardware that are installed on each host and networking device.
The interaction between the different protocols on a device can be illustrated as a protocol stack, as shown in Figure 5-6. A stack illustrates the protocols as a layered hierarchy, with each higher-level protocol depending on the services of the protocols shown in the lower levels.
Figure 5-6 A Protocol Stack for Internet Communications
The separation of functions enables each layer in the stack to operate independently of others. For example, you can use your laptop computer connected to a cable modem at home to access your favorite website, or you can view the same website on your laptop using a wireless connection at the library. The function of the web browser is not affected by the change in the physical location or the method of connectivity.
The protocols in Figure 5-6 are described as follows:
Hypertext Transfer Protocol (HTTP)—This protocol governs the way a web server and a web client interact. HTTP defines the content and formatting of the requests and responses that are exchanged between the client and server. Both the client and the web server software implement HTTP as part of the application. HTTP relies on other protocols to govern how the messages are transported between the client and server.
Transmission Control Protocol (TCP)—This protocol manages the individual conversations. TCP is responsible for guaranteeing the reliable delivery of the information and managing flow control between the end devices.
Internet Protocol (IP)—This protocol is responsible for delivering messages from the sender to the receiver. IP is used by routers to forward the messages across multiple networks.
Ethernet—This protocol is responsible for the delivery of messages from one NIC to another NIC on the same Ethernet local-area network (LAN).
Layered models help you visualize how the various protocols work together to enable network communications. A layered model depicts the operation of the protocols occurring within each layer, as well as the interaction with the layers above and below it. The layered model has many benefits:
Assists in protocol design, because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below
Fosters competition because products from different vendors can work together
Enables technology changes to occur at one level without affecting the other levels
Provides a common language to describe networking functions and capabilities
The first layered model for internetwork communications was created in the early 1970s and is referred to as the Internet model. It defines four categories of functions that must occur in order for communications to be successful. The suite of TCP/IP protocols that are used for Internet communications follows the structure of this model, as shown in Table 5-2. Because of this, the Internet model is commonly referred to as the TCP/IP model.
Table 5-2 The Layers of the TCP/IP Model
TCP/IP Model Layer
Description
Application
Represents data to the user, plus encoding and dialogue control
Transport
Supports communication between various devices across diverse networks
Internet
Determines the best path through the network
Network Access
Controls the hardware devices and media that make up the network
The OSI Reference Model (5.3.5)
Two basic types of models are used to describe the functions that must occur in order for network communications to be successful: protocol models and reference models.
Protocol model—This model closely matches the structure of a particular protocol suite. A protocol suite includes the set of related protocols that typically provide all the functionality required for people to communicate with the data network. The TCP/IP model is a protocol model because it describes the functions that occur at each layer of protocols within the TCP/IP suite.
Reference model—This type of model describes the functions that must be completed at a particular layer but does not specify exactly how a function should be accomplished. A reference model is not intended to provide a sufficient level of detail to define precisely how each protocol should work at each layer. The primary purpose of a reference model is to aid in clearer understanding of the functions and processes necessary for network communications.
The most widely known internetwork reference model was created by the Open Systems Interconnection (OSI) project at the International Organization for Standardization (ISO). It is used for data network design, operation specifications, and troubleshooting. This model is commonly referred to as the OSI model. The OSI layers are described in Table 5-3.
Just like humans, computers use rules, or protocols, to communicate. Protocols are required for computers to properly communicate across the network. In both a wired and wireless environment, a local network is defined as an area where all hosts must “speak the same language,” which in computer terms means they must “share a common protocol.”
If everyone in the same room spoke a different language, they would not be able to communicate. Likewise, if devices in a local network did not use the same protocols, they would not be able to communicate.
Networking protocols define many aspects of communication over the local network. As shown in Table 5-1, these protocols include message format, message size, timing, encoding, encapsulation, and message patterns.
Table 5-1 Protocol Characteristics
Protocol Characteristic
Description
Message format
When a message is sent, it must use a specific format or structure. Message formats depend on the type of message and the channel that is used to deliver the message.
Message size
The rules that govern the size of the pieces communicated across the network are very strict. They can also be different, depending on the channel used. When a long message is sent from one host to another over a network, breaking the message into smaller pieces might be necessary to ensure that the message can be delivered reliably.
Timing
Many network communication functions are dependent on timing. Timing determines the speed at which the bits are transmitted across the network. It also affects when an individual host can send data and the total amount of data that can be sent in any one transmission.
Encoding
Messages sent across the network are first converted into bits by the sending host. Each bit is encoded into a pattern of sounds, light waves, or electrical impulses depending on the network media over which the bits are transmitted. The destination host receives and decodes the signals to interpret the message.
Encapsulation
Each message transmitted on a network must include a header that contains addressing information that identifies the source and destination hosts; otherwise, it cannot be delivered. Encapsulation is the process of adding this information to the pieces of data that make up the message. In addition to addressing, other information in the header may ensure that the message is delivered to the correct application on the destination host.
Message pattern
Some messages require an acknowledgment before the next message can be sent. This type of request/response pattern is a common aspect of many networking protocols. However, other types of messages may be simply streamed across the network, without concern as to whether they reach their destination.
Lab—My Protocol Rules (5.1.4)
In this lab, you will complete the following objectives:
Relate computer network protocols to the rules that you use every day for various forms of communication.
Define the rules that govern how you send and interpret text messages.
Explain what would happen if the sender and receiver did not agree on the details of the protocol.
Communication standards are required in all aspects of human communications such as when addressing an envelope. There is a standard regarding the placement of the sender‛s address, destination address, and even where you put the stamp. Network communication also requires standards to ensure that all the devices in the network use the same rules to send and receive information.
With the increasing number of new devices and technologies coming online, how is it possible to manage all the changes and still reliably deliver services such as email? The answer is Internet standards.
A standard is a set of rules that determine how something must be done. Networking and Internet standards ensure that all devices connecting to the network implement the same set of rules or protocols in the same manner. Using standards, different types of devices are able to send information to each other over the Internet. For example, the way in which an email is formatted, forwarded, and received by all devices is done according to a standard. If one person sends an email via a personal computer, another person can use a mobile phone to receive and read the email as long as the mobile phone uses the same standards as the personal computer.
An Internet standard is the end result of a comprehensive cycle of discussion, problem solving, and testing. These different standards are developed, published, and maintained by a variety of organizations, as shown in Figure 5-5. When a new standard is proposed, each stage of the development and approval process is recorded in a numbered Request for Comments (RFC) document so that the evolution of the standard is tracked. RFCs for Internet standards are published and managed by the Internet Engineering Task Force (IETF).
Figure 5-5 Internet Standards Organizations
Other standards organizations that support the Internet are shown in Figure 5-5.
The number of public IPv4 addresses is severely limited, which was one of the primary reasons for RFC 1918 private IPv4 addresses. NAT for IPv4 provides for the translation between private and public IPv4 addresses.
The wireless router receives a public address from the ISP, which allows it to send and receive packets on the Internet. It, in turn, provides private addresses to local network clients. Because private addresses are not allowed on the Internet, a process is needed for translating private addresses into unique public addresses to allow local clients to communicate on the Internet.
The process used to convert private addresses to Internet-routable addresses is called Network Address Translation (NAT). With NAT, a private (local) source IPv4 address is translated to a public (global) address. The process is reversed for incoming packets. The wireless router is able to translate many internal IPv4 addresses to the same public address by using NAT.
Only packets destined for other networks need to be translated. These packets must pass through the gateway, where the wireless router replaces the private IPv4 address of the source host with its own public IPv4 address.
Although each host on the internal network has a unique private IPv4 address assigned to it, the hosts must share the single Internet-routable address assigned to the wireless router.
In Figures 10-3 and 10-4, a home router translates packets using NAT.
Figure 10-3 Wireless Router Using NAT to Translate Outbound Traffic
Figure 10-4 Wireless Router Using NAT to Translate Inbound Traffic
Packet Tracer—Examine NAT on a Wireless Router (10.2.3)
In this activity, you will complete the following objectives:
Examine NAT configuration on a wireless router.
Set up four PCs to connect to a wireless router using DHCP.
Examine traffic that crosses the network using NAT.
IPv4 was designed in the 1970s and implemented in 1980. Since then, the number of devices that access the Internet has increased substantially, beyond the 4.3 billion IPv4 addresses.
You already know that IPv4 is running out of addresses. That is why you need to learn about IPv6.
IPv6 is designed to be the successor to IPv4. IPv6 has a larger, 128-bit address space, providing 340 undecillion (that is, 340 followed by 36 zeros) possible addresses. However, IPv6 is more than just a larger address space.
When the IETF began its development of a successor to IPv4, it used this opportunity to fix the limitations of IPv4 and include enhancements. One example is Internet Control Message Protocol version 6 (ICMPv6), which includes address resolution and address autoconfiguration not found in ICMP for IPv4 (ICMPv4) and IPv6 addresses (ICMPv6).
The depletion of IPv4 address space has been the motivating factor for moving to IPv6. As Africa, Asia, and other areas of the world become more connected to the Internet, there are not enough IPv4 addresses to accommodate this growth. As shown in Figure 10-5, all five Regional Internet Registries (RIRs) have run out of IPv4 addresses.
Figure 10-5 RIR IPv4 Exhaustion Dates
As noted previously, IPv4 has a theoretical maximum of 4.3 billion addresses. Private addresses, in combination with Network Address Translation (NAT), have been instrumental in slowing the depletion of IPv4 address space. However, NAT is problematic for many applications, creates latency, and has limitations that severely impede peer-to-peer communications.
With the ever-increasing number of mobile devices, mobile providers have been leading the way with the transition to IPv6. The top two mobile providers in the United States report that over 90 percent of their traffic is over IPv6.
Most top ISPs and content providers such as YouTube, Facebook, and Netflix have also made the transition. Many companies like Microsoft, Facebook, and LinkedIn are transitioning to IPv6-only internally. In 2020, US broadband ISP Comcast reported an IPv6 deployment of over 74 percent, and the country of India is now over 62 percent.
The Internet of today is significantly different than the Internet of past decades. The Internet of today is more than email, web pages, and file transfers between computers. The evolving Internet includes an Internet of Things (IoT). No longer will the only devices accessing the Internet be computers, tablets, and smartphones. The sensor-equipped, Internet-ready devices will include everything from automobiles and biomedical devices to household appliances and lighting systems.
With an increasing Internet population, a limited IPv4 address space, issues with NAT, and the IoT, now is the time to transition to IPv6.
So far, we‛ve talked only about the existence of IPv4 addressing. This chapter explains how IPv4 and IPv6 will coexist in networks for some time to come. It shows you how an IPv6 address is structured and the benefits of IPv6 addressing over IPv4. But the fun part of this chapter is converting binary to hexadecimal notation. Don‛t know what hexadecimal notation is? Read on.
The router provides a gateway through which hosts on one network can communicate with hosts on other networks. Each interface on a router is connected to a separate network.
The IPv4 address assigned to the interface identifies which local network is connected directly to it.
Every host on a network must use the router as a gateway to other networks. Therefore, each host must know the IPv4 address of the router interface connected to the network where the host is attached. This address is known as the default gateway address. It can be either statically configured on the host or received dynamically by DHCP.
When a wireless router is configured to be a DHCP server for the local network, it automatically sends the correct interface IPv4 address to the hosts as the default gateway address. In this manner, all hosts on the network can use that IPv4 address to forward messages to hosts located at the ISP and get access to hosts on the Internet. Wireless routers are usually set to be DHCP servers by default.
The IPv4 address of that local router interface becomes the default gateway address for the host configuration. The default gateway is provided, either statically or by DHCP.
When a wireless router is configured as a DHCP server, it provides its own internal IPv4 address as the default gateway to DHCP clients. It also provides them with their respective IPv4 address and subnet mask, as shown in Figure 10-1.
The wireless router acts as a DHCP server for all local hosts attached to it, either by Ethernet cable or wirelessly. These local hosts are referred to as being located on an internal, or inside, network. Most DHCP servers are configured to assign private addresses to the hosts on the internal network rather than Internet routable public addresses. This configuration ensures that, by default, the internal network is not directly accessible from the Internet.
The default IPv4 address configured on the local wireless router interface is usually the first host address on that network. Internal hosts must be assigned addresses within the same network as the wireless router, either statically configured, or through DHCP. When configured as a DHCP server, the wireless router provides addresses in this range. It also provides the subnet mask information and its own interface IPv4 address as the default gateway, as shown in Figure 10-2.
Figure 10-2 Default Router as Both a DHCP Server and a DHCP Client
Many ISPs also use DHCP servers to provide IPv4 addresses to the Internet side of the wireless router installed at their customer sites. The network assigned to the Internet side of the wireless router is referred to as the external, or outside, network.
When a wireless router is connected to the ISP, it acts like a DHCP client to receive the correct external network IPv4 address for the Internet interface. ISPs usually provide an Internet-routable address, which enables hosts connected to the wireless router to have access to the Internet.
The wireless router serves as the boundary between the local internal network and the external Internet.
1. Which destination IPv4 address does a DHCPv4 client use to send the initial DHCP discover packet when the client is looking for a DHCP server?
255.255.255.255
127.0.0.1
The IP address of the default gateway
224.0.0.1
2. Which is a DHCPv4 address allocation method that assigns IPv4 addresses for a limited lease period?
Manual allocation
Pre-allocation
Automatic allocation
Dynamic allocation
3. Which DHCPv4 message does a client send to accept an IPv4 address that is offered by a DHCP server?
Broadcast DHCPREQUEST
Broadcast DHCPACK
Unicast DHCPOFFER
Unicast DHCPACK
4. Refer to the exhibit. A user is configuring a PC with the IP settings as displayed, but the operating system does not accept them. What is the problem?
The IP address is not a usable host address.
The DNS settings are not configured.
The subnet mask is wrong.
The gateway address is not configured.
5. Which types of devices are typically assigned static IP addresses? (Choose two.)
Printers
Laptops
Workstations
Web servers
6. A DHCP-enabled client PC has just booted. During which steps does the client PC use broadcast messages when communicating with a DHCP server? (Choose two.)
DHCPACK
DHCPNAK
DHCPOFFER
DHCPREQUEST
DHCPDISCOVER
7. Why is DHCP generally the preferred method of assigning IP addresses to hosts on large networks? (Choose two.)
It guarantees that every device that needs an address will get one.
It provides an address only to devices that are authorized to be connected to the network.
It eliminates most address configuration errors.
It reduces the burden on network support staff.
It ensures that addresses are applied only to devices that require a permanent address.
8. If more than one DHCP server is available on the local network, in which order are DHCP messages sent between a host and a DHCP server?
Request, acknowledgment, discover, offer
Request, discover, offer, acknowledgment
Discover, offer, request, acknowledgment
Acknowledgment, request, offer, discover
9. A DHCP server is used to assign IP addresses dynamically to the hosts on a network. The address pool is configured with 192.168.10.0/24. Three printers on this network need to use reserved static IP addresses from the pool. How many IP addresses in the pool are left to be assigned to other hosts?
When a device that is configured to use DHCP boots, the client broadcasts a DHCPDISCOVER message to identify any available DHCP servers on the network.
If the client receives several DHCPOFFER messages from different servers, it sends a unicast DHCPREQUEST message to the server from which it chooses to obtain IP information.
A client must wait for lease expiration before it sends another DHCPREQUEST message.
The DHCPDISCOVER message contains the IP address and subnet mask to be assigned, the IP address of the DNS server, and the IP address of the default gateway.
If you enter an airport or coffee shop with a wireless hotspot, DHCP makes it possible for you to access the Internet. As you enter the area, your laptop DHCP client contacts the local DHCP server via a wireless connection. The DHCP server assigns an IPv4 address to your laptop.
Various types of devices can be DHCP servers as long as they are running DHCP service software. With most medium to large networks, the DHCP server is usually a local dedicated PC-based server.
With home networks, the DHCP server may be located at the ISP, and a host on the home network receives its IPv4 configuration directly from the ISP, as shown in Figure 9-3. However, this is not very common.
Figure 9-3 DHCP Servers and Clients
Most home networks and small businesses use a wireless router. It is typically a single device, which is a router, a modem, a wireless access point, and an Ethernet switch. In this case, the wireless router is both a DHCP client and a server. The wireless router acts as a client to receive its IPv4 configuration from the ISP and then acts as a DHCP server for internal hosts on the local network. The router receives the public IPv4 address from the ISP, and in its role as a DHCP server, it distributes private addresses to internal hosts.
In addition to PC-based servers and wireless routers, other types of networking devices such as dedicated routers can provide DHCP services to clients, although this is not as common.
Note
DHCP for IPv6 (DHCPv6) provides similar services for IPv6 clients.
A device can receive its IPv4 addressing information dynamically from a DHCPv4 server. Most client computers, including desktop computers, laptops, smartphones, and tablets, receive their IPv4 addressing using DHCPv4.
When a host is first configured as a DHCP client, it does not have an IPv4 address, subnet mask, or default gateway. It obtains this information from a DHCP server, either on the local network or one located at the ISP. The DHCP server is configured with a range, or pool, of IPv4 addresses that can be assigned to DHCP clients.
The DHCP server may be located on another network. DHCP clients are still able to obtain IPv4 addresses as long as the routers in between are configured to forward DHCP requests.
A client that needs an IPv4 address sends a DHCP discover message, which is a broadcast with a destination IPv4 address of 255.255.255.255 (32 ones) and a destination MAC address of FF-FF-FF-FF-FF-FF (48 ones). All hosts on the network receive this broadcast DHCP frame, but only a DHCP server replies. The server responds with a DHCP offer, suggesting an IPv4 address for the client. The host then sends a DHCP request asking to use the suggested IPv4 address. The server responds with a DHCP acknowledgment, as shown in Figure 9-4.
For most home and small business networks, a wireless router provides DHCP services to the local network clients. To configure a home wireless router, you can access its graphical web interface by opening the browser and entering the router default IPv4 address 192.168.0.1 in the IP Address field, as shown in Figure 9-5 for a Packet Tracer wireless router. Home routers have a similar interface.
Figure 9-5 Packet Tracer DHCP Configuration on a Wireless Router
The IPv4 address of 192.168.0.1 and subnet mask of 255.255.255.0 are the defaults for the internal router interface. This is the default gateway for all hosts on the local network and also the internal DHCP server IPv4 address. Most home wireless routers have DHCP Server enabled by default.
On the DHCP configuration screen, a default DHCP range is available. You can also specify a starting address for the DHCP range (do not use 192.168.0.1 because the router is assigned this address) and the number of addresses to be assigned. The lease time can also be modified (the default in Figure 9-5 is 24 hours). The DHCP configuration feature on most routers gives information about connected hosts and IPv4 addresses, their associated MAC address, and lease times.
Packet Tracer—Configure DHCP on a Wireless Router (9.2.5)
In this activity, you will complete the following objectives:
Connect three PCs to a wireless router.
Change the DHCP setting to a specific network range.
Configure the clients to obtain their address via DHCP.
The following is a summary of each topic in the chapter:
Static and Dynamic Addressing—IPv4 addresses can be assigned either statically or dynamically. With a static assignment, the network administrator must manually configure the network information for a host, which minimally includes the host IPv4 address, subnet mask, and default gateway. When using static IPv4 addressing, maintain an accurate list of which IPv4 addresses are assigned to which devices. These are permanent addresses.
Dynamic addressing is done using DHCP. DHCP provides automatic assignment of addressing information such as IPv4 address, subnet mask, default gateway, and other IPv4 networking parameters. DHCP can allocate IP addresses for a configurable period of time, called a lease period. The lease period is an important DHCP setting. When the lease period expires or the DHCP server gets a DHCPRELEASE message, the address is returned to the DHCP pool for reuse.
With home networks, the DHCP server may be located at the ISP. A host on the home network receives its IPv4 configuration directly from the ISP. Many home networks and small businesses use a wireless router and modem. In this case, the wireless router is both a DHCP client and a server. The wireless router acts as a client to receive its IPv4 configuration from the ISP and then acts as a DHCP server for internal hosts on the local network.
Many networks use both DHCP and static addressing. DHCP is used for general-purpose hosts, such as end-user devices. Static addressing is used for network devices, such as gateway routers, switches, servers, and printers.
DHCPv6 provides similar services for IPv6 clients. One important difference is that DHCPv6 does not provide a default gateway address. This can only be obtained dynamically from the Router Advertisement message of the router.
DHCPv4 Configuration—The DHCP server is configured with a range, or pool, of IPv4 addresses that can be assigned to DHCP clients. A client that needs an IPv4 address sends a DHCP discover message, which is a broadcast with a destination IPv4 address of 255.255.255.255 (32 ones) and a destination MAC address of FF-FF-FF-FF-FF-FF (48 ones). All hosts on the network receive this broadcast DHCP frame, but only a DHCP server replies. The server responds with a DHCP offer, suggesting an IPv4 address for the client. The host then sends a DHCP request asking to use the suggested IPv4 address. The server responds with a DHCP acknowledgment.
The IPv4 address of 192.168.1.1 and subnet mask of 255.255.255.0 are examples of a router‛s LAN interface. This is the default gateway for all hosts on the local network and also the internal DHCP server IPv4 address. The DHCP configuration feature on most Cisco Integrated Services Routers (ISRs) provides information about connected hosts and IPv4 addresses, their associated MAC address, and lease times.
