CompTIA Network+ N10-006 Cert Guide (2015)
Chapter 1. Computer Network Fundamentals
After completion of this chapter, you will be able to answer the following questions:
What is the purpose of a network?
What are some examples of network components?
How are networks defined by geography?
How are networks defined by topology?
How are networks defined by resource location?
What comes to mind when you think of a computer network? Is it the Internet? Is it e-mail? Is it the wireless connection that lets you print to your printer from your laptop?
Whatever your current perception of a computer network, this chapter and book, as a whole, help you gain deep appreciation and understanding of networked computing. Be aware that although we commonly think of computer networks as interconnecting computers, today computer networks interconnect a variety of devices in addition to just computers. Examples include game consoles, video-surveillance devices, IP-based telephones, tablets, and smartphones. Therefore, throughout this book, you can think of the term computer network as being synonymous with the more generic term network, because these terms will be used interchangeably.
In this chapter, the goal is to acquaint you with the purpose of a network and help you categorize a given network based on criteria such as geography, topology, and the location of a network’s resources. An implied goal of this and all other chapters in this book is to prepare you to successfully pass the CompTIA Network+ exam, which is considered to be a cornerstone exam in the information technology (IT) industry.
Foundation Topics
Defining a Network
In the movie Field of Dreams, you heard, “If you build it, they will come.” That statement most certainly applies to the evolution of network-based services seen in modern-day networks. Computer networks are no longer relegated to allowing a group of computers to access a common set of files stored on a computer designated as a file server. Instead, with the building of high-speed, highly redundant networks, network architects are seeing the wisdom of placing a variety of traffic types on a single network. Examples include voice and video, in addition to data.
One could argue that a network is the sum of its parts. So, as you begin your study of networking, you should grasp a basic understanding of fundamental networking components. These components include such entities as a client, server, hub, switch, router, and the media used to interconnect these devices.
The Purpose of Networks
At its essence, a network’s purpose is to make connections. These connections might be between a PC and a printer or between a laptop and the Internet, as just a couple of examples. However, the true value of a network comes from the traffic flowing over those connections. Consider a sampling of applications that can travel over a network’s connections:
File sharing between two computers
Video chatting between computers located in different parts of the world
Surfing the web (for example, to use social media sites, watch streaming video, listen to an Internet radio station, or do research for a school term paper)
Instant messaging (IM) between computers with IM software installed
E-mail
Voice over IP (VoIP), to replace traditional telephony systems
A term commonly given to a network transporting multiple types of traffic (for example, voice, video, and data) is a converged network. A converged network might offer significant cost savings to organizations that previously supported separate network infrastructures for voice, data, and video traffic. This convergence can also potentially reduce staffing costs, because only a single network needs to be maintained, rather than separate networks for separate traffic types.
Overview of Network Components
Designing, installing, administering, and troubleshooting a network requires the ability to recognize various network components and their functions. Although this is the focus of Chapter 3, “Network Components,” before we can proceed much further, you need a basic working knowledge of how individual components come together to form a functioning network.
The components to consider for now are client, server, hub, switch, router, media, and wide-area network (WAN) link. As a reference for this discussion, consider Figure 1-1.
Figure 1-1 Sample Computer Network
The following list describes the network components depicted in Figure 1-1 and the functions they serve:
Client: The term client defines the device an end user uses to access a network. This device might be a workstation, laptop, smartphone with wireless capabilities, or a variety of other end-user terminal devices.
Server: A server, as the name suggests, serves up resources to a network. These resources might include e-mail access as provided by an e-mail server, web pages as provided by a web server, or files available on a file server.
Hub: A hub is an older technology that interconnects network components, such as clients and servers. Hubs vary in their number of available ports. However, for scalability, hubs can be interconnected, up to a point. If too many hubs are chained together, network errors can result. As discussed further in Chapter 3, a hub is a Layer 1 device and does not perform any inspection of the traffic it passes. Rather, a hub simply receives traffic in a port(that is, a receptacle to which a network cable connects) and repeats that traffic out all of the other ports.
Switch: Like a hub, a switch interconnects network components, and they are available with a variety of port densities. However, unlike a hub, a switch does not simply take traffic in on one port and blast that traffic out all other ports. Rather, a switch learns which devices reside off of which ports. As a result, when traffic comes in a switch port, the switch interrogates the traffic to see where it is destined. Then, based on what the switch has learned, the switch forwards the traffic out of the appropriate port, and not out all the other ports. This dramatically cuts down on the volume of traffic coursing through your network. A switch is considered a Layer 2 device, which means that it makes its forwarding decisions based on addresses that are physically burned into a network interface card (NIC) installed in a host (that is, any device that transmits or receives traffic on a network). This burned-in address is a Media Access Control (MAC) address.
Router: As discussed in Chapter 3, a router is considered to be a Layer 3 device, which means that it makes its forwarding decisions based on logical network addresses. Most modern networks use Internet Protocol (IP) addressing. Therefore, most routers know what logical IP networks reside off of which router interfaces. Then, when traffic comes into a router, the router examines the destination IP address of the traffic and, based on the router’s database of networks (that is, the routing table), the router intelligently forwards the traffic out the appropriate interface.
Media: The previously mentioned devices need to be interconnected via some sort of media. This media could be copper cabling. It could be a fiber-optic cable. Media might not even be a cable, as is the case with wireless networks, where radio waves travel through the media of air. Chapter 3 expands on this discussion of media. For now, realize that media varies in its cost, bandwidth capacity, and distance limitation. For example, although fiber-optic cabling is more expensive than unshielded twisted-pair cabling, it can typically carry traffic over longer distances and has a greater bandwidth capacity (that is, the capacity to carry a higher data rate).
WAN link: Today, most networks connect to one or more other networks. For example, if your company has two locations, and those two locations are interconnected (perhaps via a Frame Relay or Multiprotocol Label Switching [MPLS] network), the link that interconnects those networks is typically referred to as a wide-area network (WAN) link. WANs, and technologies supporting WANs, are covered in Chapter 7, “Wide-Area Networks.”
Networks Defined by Geography
As you might be sensing at this point, not all networks look the same. They vary in numerous ways. One criterion by which we can classify networks is how geographically dispersed the networks components are. For example, a network might interconnect devices within an office, or a network might interconnect a database at a corporate headquarters location with a remote sales office located on the opposite side of the globe.
Based on the geographic dispersion of network components, networks can be classified into various categories, including the following:
Local-area network (LAN)
Wide-area network (WAN)
Campus-area network (CAN)
Metropolitan-area network (MAN)
Personal-area network (PAN)
The following sections describe these different classifications of networks in more detail.
LAN
A LAN interconnects network components within a local area (for example, within a building). Examples of common LAN technologies you are likely to encounter include Ethernet (that is, IEEE 802.3) and wireless networks (that is, IEEE 802.11). Figure 1-2 illustrates an example of a LAN.
Figure 1-2 Sample LAN Topology
Note
IEEE stands for the Institute of Electrical and Electronics Engineers, and it is an internationally recognized standards body.
WAN
A WAN interconnects network components that are geographically separated. For example, a corporate headquarters might have multiple WAN connections to remote office sites. Multiprotocol Label Switching (MPLS), Asynchronous Transfer Mode (ATM), and Frame Relay are examples of WAN technologies. Figure 1-3 depicts a simple WAN topology, which interconnects two geographically dispersed locations.
Figure 1-3 Sample WAN Topology
Other Categories of Networks
Although LANs and WANs are the most common terms used to categorize computer networks based on geography, other categories include campus-area network (CAN), metropolitan-area network (MAN), and personal-area network (PAN).
CAN
Years ago, I was a network manager for a university. The university covered several square miles and had several dozen buildings. Within many of these buildings was a LAN. However, those building-centric LANs were interconnected. By interconnecting these LANs, another network type was created, a CAN. Besides an actual university campus, a CAN might also be found in an industrial park or business park.
MAN
More widespread than a CAN and less widespread than a WAN, a MAN interconnects locations scattered throughout a metropolitan area. Imagine, for example, that a business in Chicago has a location near O’Hare Airport, another location near the Navy Pier, and another location in the Willis Tower (previously known as the Sears Tower). If a service provider could interconnect those locations using a high-speed network, such as a 10-Gbps (that is, 10 billion bits per second) network, the interconnection of those locations would constitute a MAN. One example of a MAN technology is Metro Ethernet.
PAN
A PAN is a network whose scale is even smaller than a LAN. For example, a connection between a PC and a digital camera via a universal serial bus (USB) cable could be considered a PAN. Another example is a PC connected to an external hard drive via a FireWire connection. A PAN, however, is not necessarily a wired connection. A Bluetooth connection between your cell phone and your car’s audio system is considered a wireless PAN (WPAN). The main distinction of a PAN, however, is that its range is typically limited to just a few meters.
Networks Defined by Topology
In addition to classifying networks based on the geographic placement of their components, another approach to classifying a network is to use the network’s topology. Looks can be deceiving, however. You need to be able to distinguish between a physical topology and a logical topology.
Physical Versus Logical Topology
Just because a network appears to be a star topology (that is, where the network components all connect back to a centralized device, such as a switch), the traffic might be flowing in a circular pattern through all the network components attached to the centralized device. The actual traffic flow determines the logical topology, while the way components are physically interconnected determines the physical topology.
For example, consider Figure 1-4. The figure shows a collection of computers connected to a Token Ring media access unit (MAU). From a quick inspection of Figure 1-4, you can conclude that the devices are physically connected in a star topology, where the connected devices radiate out from a centralized aggregation point (that is, the MAU in this example).
Figure 1-4 Physical Star Topology
Next, contrast the physical topology in Figure 1-4 with the logical topology illustrated in Figure 1-5. Although the computers physically connect to a centralized MAU, when you examine the flow of traffic through (or in this case, around) the network, you see that the traffic flow actually loops round-and-round the network. The traffic flow dictates how to classify a network’s logical topology. In this instance, the logical topology is a ring topology because the traffic circulates around the network as if circulating around a ring.
Figure 1-5 Logical Ring Topology
Although Token Ring, as used in this example, is rarely seen in modern networks, it illustrates how a network’s physical and logical topologies can differ significantly.
Bus Topology
A bus topology, as depicted in Figure 1-6, typically uses a cable running through the area requiring connectivity. Devices that need to connect to the network then tap into this nearby cable. Early Ethernet networks commonly relied on bus topologies.
Figure 1-6 Bus Topology
A network tap might be in the form of a T connector (commonly used in older 10BASE2 networks) or a vampire tap (commonly used in older 10BASE5 networks). Figure 1-7 shows an example of a T connector.
Figure 1-7 T Connector
Note
The Ethernet standards mentioned here (that is, 10BASE2 and 10BASE5), in addition to many other Ethernet standards, are discussed in detail in Chapter 4, “Ethernet Technology.”
A bus and all devices connected to that bus make up a network segment. As discussed in Chapter 4, a single network segment is a single collision domain, which means that all devices connected to the bus might try to gain access to the bus at the same time, resulting in an error condition known as a collision. Table 1-1 identifies some of the primary characteristics, benefits, and drawbacks of a bus topology.
Table 1-1 Characteristics, Benefits, and Drawbacks of a Bus Topology
Ring Topology
Figure 1-8 offers an example of a ring topology, where traffic flows in a circular fashion around a closed network loop (that is, a ring). Typically, a ring topology sends data, in a single direction, to each connected device in turn, until the intended destination receives the data. Token Ring networks typically relied on a ring topology, although the ring might have been the logical topology, whereas physically, the topology was a star topology.
Figure 1-8 Ring Topology
Token Ring, however, was not the only popular ring-based topology popular in networks back in the 1990s. Fiber Distributed Data Interface (FDDI) was another variant of a ring-based topology. Most FDDI networks (which, as the name suggests, have fiber optics as the media) used not just one ring, but two. These two rings sent data in opposite directions, resulting in counter-rotating rings. One benefit of counter-rotating rings was that if a fiber broke, the stations on each side of the break could interconnect their two rings, resulting in a single ring capable of reaching all stations on the ring.
Because a ring topology allows devices on the ring to take turns transmitting on the ring, contention for media access was not a problem, as it was for a bus topology. If a network had a single ring, however, the ring became a single point of failure. If the ring were broken at any point, data would stop flowing. Table 1-2 identifies some of the primary characteristics, benefits, and drawbacks of a ring topology.
Table 1-2 Characteristics, Benefits, and Drawbacks of a Ring Topology
Star Topology
Figure 1-9 shows a sample star topology with a hub at the center of the topology and a collection of clients individually connected to the hub. Notice that a star topology has a central point from which all attached devices radiate. In LANs, that centralized device was typically a hub back in the early 1990s. Modern networks, however, usually have a switch located at the center of the star.
Figure 1-9 Star Topology
Note
Chapter 3 discusses UTP and other types of cabling.
The star topology is the most popular physical LAN topology in use today, with an Ethernet switch at the center of the star and unshielded twisted-pair cable (UTP) used to connect from the switch ports to clients.
Table 1-3 identifies some of the primary characteristics, benefits, and drawbacks of a star topology.
Table 1-3 Characteristics, Benefits, and Drawbacks of a Star Topology
Hub-and-Spoke Topology
When interconnecting multiple sites (for example, multiple corporate locations) via WAN links, a hub-and-spoke topology has a WAN link from each remote site (that is, a spoke site) to the main site (that is, the hub site). This approach, an example of which is shown in Figure 1-10, is similar to the star topology used in LANs.
Figure 1-10 Hub-and-Spoke Topology
With WAN links, a service provider is paid a recurring fee for each link. Therefore, a hub-and-spoke topology helps minimize WAN expenses by not directly connecting any two spoke locations. If two spoke locations need to communicate between themselves, their communication is sent via the hub location. Table 1-4 contrasts the benefits and drawbacks of a hub-and-spoke WAN topology.
Table 1-4 Characteristics, Benefits, and Drawbacks of a Hub-and-Spoke WAN Topology
Full-Mesh Topology
Although a hub-and-spoke WAN topology lacked redundancy and suffered from suboptimal routes, a full-mesh topology, as shown in Figure 1-11, directly connects every site to every other site.
Figure 1-11 Full-Mesh Topology
Because each site connects directly to every other site, an optimal path can be selected, as opposed to relaying traffic via another site. Also, a full-mesh topology is highly fault tolerant. By inspecting Figure 1-11, you can see that multiple links in the topology could be lost, and every site might still be able to connect to every other site. Table 1-5 summarizes the characteristics of a full-mesh topology.
Table 1-5 Characteristics, Benefits, and Drawbacks of a Full-Mesh WAN Topology
Partial-Mesh Topology
A partial-mesh WAN topology, as depicted in Figure 1-12, is a hybrid of the previously described hub-and-spoke topology and full-mesh topology. Specifically, a partial-mesh topology can be designed to provide an optimal route between selected sites, while avoiding the expense of interconnecting every site to every other site.
Figure 1-12 Partial-Mesh Topology
When designing a partial-mesh topology, a network designer must consider network traffic patterns and strategically add links interconnecting sites that have higher volumes of traffic between themselves. Table 1-6 highlights the characteristics, benefits, and drawbacks of a partial-mesh topology.
Table 1-6 Characteristics, Benefits, and Drawbacks of a Partial-Mesh Topology
Networks Defined by Resource Location
Yet another way to categorize networks is based on where network resources reside. An example of a client/server network is a collection of PCs all sharing files located on a centralized server. However, if those PCs had their operating system (OS) (for example, Microsoft Windows 8 or Mac OS X) configured for file sharing, they could share files from one another’s hard drives. Such an arrangement would be referred to as a peer-to-peer network, because the peers (that is, the PCs in this example) make resources available to other peers. The following sections describe client/server and peer-to-peer networks in more detail.
Client/Server Networks
Figure 1-13 illustrates an example of a client/server network, where a dedicated file server provides shared access to files, and a networked printer is available as a resource to the network’s clients. Client/server networks are commonly used by businesses. Because resources are located on one or more servers, administration is simpler than trying to administer network resources on multiple peer devices.
Figure 1-13 Client/Server Network Example
The performance of a client/server network can be better than that of a peer-to-peer network because resources can be located on dedicated servers rather than on a PC running a variety of end-user applications. Backups can be simplified because fewer locations must be backed up. However, client/server networks come with the extra expense of dedicated server resources. Table 1-7 contrasts the benefits and drawbacks of client/server networks.
Table 1-7 Characteristics, Benefits, and Drawbacks of a Client/Server Network
Note
A server in a client/server network could be a computer running a network operating system (NOS), such as Linux Server or a variety of Microsoft Windows Server operating systems. Alternatively, a server might be a host making its file system available to remote clients via the Network File System (NFS) service, which was originally developed by Sun Microsystems.
Note
A variant of the traditional server in a client/server network, where the server provides shared file access, is network-attached storage (NAS). A NAS device is a mass storage device that attaches directly to a network. Rather than running an advanced NOS, a NAS device usually makes files available to network clients via a service such as NFS.
Peer-to-Peer Networks
Peer-to-peer networks allow interconnected devices (for example, PCs) to share their resources with one another. Those resources could be, for example, files or printers. As an example of a peer-to-peer network, consider Figure 1-14, where each of the peers can share files on their own hard drives, and one of the peers has a directly attached printer that can be shared with the other peers in the network.
Figure 1-14 Peer-to-Peer Network Example
Peer-to-peer networks are commonly seen in smaller businesses and in homes. The popularity of these peer-to-peer networks is fueled in part by client operating systems that support file and print sharing. Scalability for peer-to-peer networks is a concern, however. Specifically, as the number of devices (that is, peers) increases, the administration burden increases. For example, a network administrator might have to manage file permissions on multiple devices, as opposed to a single server. Consider the characteristics of peer-to-peer networks as presented in Table 1-8.
Table 1-8 Characteristics, Benefits, and Drawbacks of a Peer-to-Peer Network
Note
Some networks have characteristics of both peer-to-peer and client/server networks. For example, all PCs in a company might point to a centralized server for accessing a shared database in a client/server topology. However, these PCs might simultaneously share files and printers between one another in a peer-to-peer topology. Such a network, which has a mixture of client/server and peer-to-peer characteristics, is called a hybrid network.
Real-World Case Study
Acme Inc.’s headquarters is located in offices on the same floor of a building downtown. It also has two branch offices that are in remote locations. The company wants to be able to share files and do instant messaging, e-mail, and voice all on its own private networks where possible. It also wants connectivity to the Internet. To accomplish this, it will set up a LAN with UTP cabling, with the clients and servers connecting to a switch in a physical star topology. For connectivity between the headquarters office and the two branches, the company will use the services of a service provider for the WAN connectivity. The service provider will provide logical point-to-point connections between the headquarters office and each branch, but physically the path between the headquarters of the branch offices is going through several routers over the service provider network. For the time being, branch 1 and branch 2 will not have direct connectivity between each other, but can forward traffic between each other through the headquarters site. Next year, as more funds are available, the company can purchase WAN connectivity directly between branch 1 and branch 2. With that added connectivity between each location and all other sites, the company will have a full mesh.
Summary
The main topics covered in this chapter are the following:
You were introduced to various network components, including client, server, hub, switch, router, media, and WAN link.
One way to classify networks is by their geographical dispersion. Specifically, these network types were identified: LAN, WAN, CAN, MAN, and PAN.
Another approach to classifying networks is based on a network’s topology. Examples of network types, based on topology, include bus, ring, star, partial mesh, full mesh, and hub and spoke.
This chapter contrasted client/server and peer-to-peer networks.
Exam Preparation Tasks
Review All the Key Topics
Review the most important topics from inside the chapter, noted with the Key Topic icon in the outer margin of the page. Table 1-9 lists these key topics and the page numbers where each is found.
Table 1-9 Key Topics for Chapter 1
Complete Tables and Lists from Memory
Print a copy of Appendix D, “Memory Tables” (found on the DVD), or at least the section for this chapter, and complete as many of the tables as possible from memory. Appendix E, “Memory Table Answer Key,” also on the DVD, includes the completed tables and lists so you can check your work.
Define Key Terms
Define the following key terms from this chapter, and check your answers in the Glossary:
client
server
hub
switch
router
media
WAN link
local-area network (LAN)
wide-area network (WAN)
campus-area network (CAN)
metropolitan-area network (MAN)
personal-area network (PAN)
logical topology
physical topology
bus topology
ring topology
star topology
hub-and-spoke topology
full-mesh topology
partial-mesh topology
client/server network
peer-to-peer network
Complete Chapter 1 Hands-On Lab in Network+ Simulator Lite
Network Topologies
Review Questions
The answers to these review questions appear in Appendix A, “Answers to Review Questions.”
1. Which of the following is a device directly used by an end user to access a network?
a. Server
b. LAN
c. Client
d. Router
2. Which device makes traffic-forwarding decisions based on MAC addresses?
a. Hub
b. Router
c. Switch
d. Multiplexer
3. A company has various locations in a city interconnected using Metro Ethernet connections. This is an example of what type of network?
a. WAN
b. CAN
c. PAN
d. MAN
4. A network formed by interconnecting a PC to a digital camera via a USB cable is considered what type of network?
a. WAN
b. CAN
c. PAN
d. MAN
5. Which of the following physical LAN topologies requires the most cabling?
a. Bus
b. Ring
c. Star
d. WLAN
6. Which of the following topologies offers the highest level of redundancy?
a. Full mesh
b. Hub and spoke
c. Bus
d. Partial mesh
7. How many WAN links are required to create a full mesh of connections between five remote sites?
a. 5
b. 10
c. 15
d. 20
8. Identify two advantages of a hub-and-spoke WAN topology as compared to a full-mesh WAN topology.
a. Lower cost
b. Optimal routes
c. More scalable
d. More redundancy
9. Which type of network is based on network clients sharing resources with one another?
a. Client/server
b. Client-peer
c. Peer-to-peer
d. Peer-to-server
10. Which of the following is an advantage of a peer-to-peer network, as compared with a client/server network?
a. More scalable
b. Less expensive
c. Better performance
d. Simplified administration