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From the author of Sybex’s best-selling CCNA: Cisco Certified Network Associate Study Guide comes the streamlined tool you need to hone in on critical CCNA information: CCNA Fast Pass. The enclosed CD lets you practice, ...
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From the author of Sybex’s best-selling CCNA: Cisco Certified Network Associate Study Guide comes the streamlined tool you need to hone in on critical CCNA information: CCNA Fast Pass. The enclosed CD lets you practice, practice, practice so you can approach the exam with confidence.
Designing internetworks using Cisco technology
Developing an access List
Evaluating TCP/IP communication process
Configuring routers and switches
Configuring IP addresses, subnet masks, and gateway addresses
Performing LAN, VLAN, and WAN troubleshooting
Understanding rules for packet control
Featured on the CD
The enclosed CD features two CCNA practice exams covering all exam objectives. You also get a set of electronic flash cards for PCs, Pocket PCs, and Palm handhelds, plus a glossary of key terms.
About the Author
Todd Lammle, CCNA, CCNP, has over twenty years of experience working with various LAN and WANs, and has been working on Cisco router networks since 1986. He is CEO and Chief Scientist of RouterSim, LLC and President of GlobalNet Training, Inc. Todd was voted "Best Study Guide Author" in the CertCities 2002 Readers' Choice Awards.
CISCO CCNA EXAM GUIDELINES COVERED IN THIS CHAPTER:
A large part of the CCNA exam deals with not just the configuration, but the work that comes before you actually log into the router for setup and troubleshooting. This chapter addresses those issues. We will discuss the process of designing networks, and making decisions about issues such as which devices, IP addressing, and routing protocols to choose. Let's face it, if you don't have a handle on these decisions, how can you even order equipment?
Let's get started by looking first at a simple LAN and choosing which technologies to include.
1.1 Designing a Simple LAN Using Cisco Technology
You can substitute a number of interchangeable terms for local area network (LAN), depending on the context (these terms will be covered in more detail later in the chapter). They include the following:
* Broadcast domain, which is used in the context of Layer 2 vs. Layer 1 segmentation
* Subnet or network, which are used in the context of IP networking
* Data Link (Layer 2 from the OSI model)
* Virtual LAN (VLAN), which is used in the context of creating broadcast domains in switched Ethernet environments
Why discuss a simple LAN? Well, it is the basis of every internetwork. An internetwork is a collection of connected LANs. You can create an individual LAN using a variety of devices and techniques, including switches, routers, and hubs. These devices connect the hosts on the LAN to each other, and they connect the LAN to the other LANs, forming the internetwork.
The number of networks and the necessity of networking have grown exponentially over the last 15 years-and understandably so. They've had to evolve at light speed just to keep up with huge increases in basic mission-critical user needs like sharing data and printers, as well as more advanced demands like video conferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), it is a challenge to connect the relevant and sometimes numerous networks so that all users can share the networks' wealth.
It's likely that at some point, you'll have to break up one large network into a number of smaller ones because user response has dwindled to a trickle as networks grew and grew and LAN traffic congestion reached overwhelming proportions. Congestion is a really big problem. Some possible causes of LAN traffic congestion are:
* Too many hosts in a broadcast domain
* Excessive Broadcasts
* Low or insufficient bandwidth
You can help solve the congestion issue by breaking up a larger network into a number of smaller networks. This is known as network segmentation. Network segmentation is accomplished using routers, switches, and bridges.
You use routers to connect networks and route packets of data from one network to another. Cisco became the de facto standard of routers because of their high-quality router products, their great selection, and their fantastic customer service.
Routers, by default, break up a broadcast domain, which is the set of all the devices on a network segment that hear all the broadcasts sent on that segment. Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast-that is, unless you've got a router. When the router's interface receives this broadcast, it can respond by basically saying, "Thanks, but no thanks"; it can then discard the broadcast without forwarding it on to other networks.
Even though routers are known for breaking up broadcast domains by default, it's important to remember that they also break up collision domains as well.
Here are two ways that using routers in your network can reduce congestion:
* They don't forward broadcasts by default (switches and bridges do) _ * They can filter the network based on Layer 3 information (that is, based on IP address); switches and bridges cannot.
Conversely, LAN switches aren't used to create internetworks-they're employed to add functionality to a LAN. The main purpose of a switch is to make a LAN work better-to optimize its performance-by providing more bandwidth for the LAN's users. And switches don't forward packets to other networks like routers do; instead, they only forward frames from one port to another within the switched network. Switches cannot forward frames between networks; they can only carry frames to routers to be forwarded to other networks by the router.
Switches and switching technologies are covered in more detail in Chapter 4, section 4.3, Compare and contrast key characteristics of LAN environments.
By default, switches break up collision domains. Collision domain is an Ethernet term used to describe the following network scenario. One particular device sends a packet on a network segment, forcing every other device on that segment to pay attention to it. At the same time, a different device tries to transmit, which leads to a collision, after which both devices must retransmit, one at a time. Not good-very inefficient! You'll typically find this situation in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.
Switches create separate collision domains, but only one broadcast domain. Routers create separate broadcast domains.
The term bridging was introduced before routers and hubs were implemented, so it's pretty common to hear people referring to bridges as switches. That's because bridges and switches basically do the same thing-they break up collision domains on a LAN. So what this means is that a switch is basically just a multiple port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only have two or four ports. Yes, you can get your hands on a bridge with up to 16 ports, but that's nothing compared to the hundreds available on some switches!
You should use a bridge in a network where you want to reduce collisions within broadcast domains and increase the number of collision domains in your network. In this situation, bridges provide more bandwidth for users.
The Router, Switch, and Bridge Working Together
Now it's time to see how the router, switch, and bridge operate together. Figure 1.1 shows how a network looks with all of these internetwork devices in place.
Remember that the router breaks up broadcast domains for every LAN inter- face, but it also breaks up collision domains as well.
When you look at Figure 1.1, do you notice that the router is at center stage and that it connects each physical network? In this situation, I had to use this layout because of the older technologies involved-bridges and hubs. But once you have only switches in your network, things can change a lot! In the new network, you could place the LAN switches at the center of the network world and use the routers to connect only the logical networks together. If you've implemented this kind of setup, you've created virtual LANs (VLANs).
Okay, now refer back to Figure 1.1: In the top network, I used a bridge to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, this bridge only creates two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is actually pretty lame, but it's still better than having one collision domain for all your hosts!
Although bridges are used to segment networks, they will not isolate broadcast or multicast packets.
Notice something else: the three interconnected hubs at the bottom of the figure also connect to the router. This creates one humongous collision domain and one humongous broadcast domain-a messy situation, true. This makes the bridged network look much better indeed!
The best network connected to the router is the LAN switch network on the left. Why? Because, each port on that switch breaks up collision domains. But it's not all good-all the devices are still in the same broadcast domain. Remember why this can be a bad thing? Because all devices must listen to all broadcasts transmitted, and if your broadcast domains are too large, the users must process additional, and sometimes excessive, broadcasts.
Obviously, the best network is one that's correctly configured to meet the business requirements of the company it serves. LAN switches with routers, when correctly placed in the network, are the best network design.
Understand the different terms used to describe a LAN. A LAN is basically the same thing as a VLAN, subnet or network, broadcast domain, or data link. These terms all describe roughly the same concept in different contexts. A broadcast domain is used when describing segmenting with routers, a subnet or network functions in IP networking, a data link defines Layer 2 boundaries of the OSI model, and you use a VLAN when you create broadcast domains in switched Ethernet environments.
Understand which devices create a LAN and which separate and connect LANs. Switches and bridges are used to create LANs. Although they do separate collision domains, they do not create separate LANs (a collision domain and a LAN are not the same concept). Routers are used to separate LANs and connect LANs (broadcast domains).
1.2 Designing an IP Addressing Scheme to Meet Design Requirements
An IP address is a numeric identifier that is assigned to each machine on an IP network, and it designates the specific location of a device on that network. An IP address is a software address, not a hardware address-the latter is hardcoded on a network interface card (NIC) and is used for finding hosts on a local network. IP addressing was designed to allow a host on one network to communicate with a host on a different network, regardless of the type of LANs the hosts are participating in.
There are many items to consider when you go to design an IP addressing scheme because IP addressing is, well, a large topic. However, some aspects, when considered at design time, can save you significant maintenance time over the life of an internetwork. Here, I'll introduce you to some basic terminology and the hierarchical IP address system; you'll also look at private IP addresses and network address translation (NAT).
The following are several important terms vital to your understanding of the Internet Protocol (IP):
Bit A bit is one digit; either a 1 or a 0.
Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this section, always assume a byte is 8 bits.
Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.
Network address The network address is the designation used in routing to send packets to a remote network-for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.
Broadcast address This type of address is used by applications and hosts to send information to all nodes on a network. Examples include 255.255.255.255, which is all networks, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.
The Hierarchical IP Addressing Scheme
An IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, and each contains 1 byte (8 bits). You can depict an IP address using one of three methods:
* Dotted-decimal, as in 172.16.30.56
* Binary, as in 10101100.00010000.00011110.00111000
* Hexadecimal (hex for short), as in AC.10.1E.38
All these examples represent the same IP address. Hex isn't used as often as dotted-decimal or binary when IP addressing is being discussed, but you still might find an IP address stored in hex in some programs. The Windows Registry is a good example of a program that stores a machine's IP address in hex.
The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. Although you can use either type of addressing scheme, I'd advise that you use hierarchical addressing. The advantage of using a hierarchical address is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position-either 0 or 1-gives you [2.sup.32], or 4,294,967,296). The disadvantage of the flat addressing scheme and the reason it's not used for IP addressing relates to routing. If every address were unique, all routers on the Internet would need to store the address of every machine on the Internet. This would make efficient routing impossible, even if only a fraction of the possible addresses were used.
You can solve this problem by using a two- or three-level hierarchical addressing scheme that is structured by network and host, or network, subnet, and host.
This two- or three-level scheme is comparable to a telephone number. In a phone number, the first section, the area code, designates a very large area. The second section, the prefix, narrows the scope to a local calling area. The final segment, the customer number, zooms in on the specific connection. IP addresses use the same type of layered structure. Rather than all 32 bits being treated as a unique identifier, as would be the case in flat addressing, a part of the address is designated as the network address, and the other part is designated as either the subnet and host, or just the host address.
The network address (also called network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.16.30.56, for example, 172.16 is the network address.
The node address is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine-an individual-as opposed to a network, which is a group. This number can also be referred to as a host address. In the sample IP address 172.16.30.56, 30.56 is the node address.
The designers of the Internet decided to create classes of networks based on network size. For the small number of networks that possess a very large number of nodes, they created the Class A network. At the other extreme is the Class C network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the Class B network.
How you should subdivide an IP address into a network and node address is determined by the class designation of your network. Figure 1.2 summarizes the three classes of networks-a subject I'll explain in much greater detail throughout this section.
To ensure efficient routing, Internet designers defined a mandate for the leading-bits section of the address for each different network class. For example, since a router knows that a Class A network address always starts with a 0, the router might be able to speed a packet on its way after reading only the first bit of its address. This is where the address schemes define the difference between a Class A, Class B, and Class C address.
Excerpted from CCNA Fast Pass by Todd Lammle Excerpted by permission.
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|Ch. 2||Implementation & Operation||59|