Charles M. Kozierok The TCP-IP Guide

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Types and Sizes of Networks

One of the reasons that understanding networks can be difficult at times is that there are so

many different types! When someone talks about a “network”, this can mean anything from

two computers hooked together in an apartment to a globe-spanning entity with millions of

nodes. Every network is unique, and each one has an important role to play in filling the

communication and data-sharing needs of different individuals and organizations. In fact,

the great diversity and flexibility of networking is one of its most important strengths.

In this section I describe the major types of networks that exist by drawing distinctions

between them based on their size and scope, and show how each type and size is used. I

begin with a discussion of LANs, WLANs and WANs, and a few variations on these three

main categories. I explore the many terms that are related to the various sizes of networks

and how they are used, including segments, subnetworks, internetworks, intranets and

extranets

Note: As with other networking fundamentals discussions, if you have some

background or experience in networking, you may not need to read everything in

this section. I'd suggest scanning the headings of the various topics here to get a

handle for what you feel you need to read.

Local Area Networks (LANs), Wireless LANs (WLANs) and Wide

Area Networks (WANs) and Variants (CANs, MANs and PANs)

Two of the most basic ways that networks are differentiated and contrasted are the relative

distances between the devices that they connect, and the general mechanisms used to

communicate between them. The reason for making these distinctions is that the techno-

logical needs of a network differ greatly depending on the amount of ground you are trying

to cover, and also on the overall way that you want to transmit and receive information.

Fundamental Network Classifications

Many people, including me, like to divide the many kinds of networks in existence into three

general classes:

☯ Local Area Networks (LANs): Networks that connect together computers that are

relatively close to each other—generally, within the same room or building. When most

people think about networking PCs and other small computers, this is what they

usually have in mind. The vast majority of regular LANs connect using cables, so the

term “LAN” by itself usually implies a wired LAN, but not always.

☯ Wireless Local Area Networks (Wireless LANs or WLANs): Local area networks

that connect devices without wires, using radio frequencies or light. WLANs can be

entirely wireless, but most are not: they usually connect wireless devices to each other

and also to the wired portion of the network. Due to the limits of most wireless technol-

ogies, wireless LANs usually connect devices that are very close to each other,

generally within a few hundred feet at most.

☯ Wide Area Networks (WANs): Networks that connect together devices or other

networks over a greater distance than is practical for local area networking. If the

distance between devices can be measured in miles, you will generally use WAN and

not LAN technology to link them.

More often than not, WANs are used to link together physically distant LANs. For

example, a company with locations in two different cities would normally set up a LAN

in each building and then connect them together in a WAN. I also consider most

Internet access technologies to be a form of wide area networking, though some might

not agree with that. There is also the term wireless WAN (WWAN), which just refers to

a WAN that uses wireless technology.

Difficulties in Categorizing Network Classes

As with most other distinctions and categorizations in the world of networking, the lines

between these various definitions are not very concrete. As I mentioned already, wireless

LANs are usually not entirely wireless, because they contained wired elements. Similarly,

trying to say absolutely when a network is “local” and when it is “wide” is difficult.

It's also somewhat pointless to spend too much energy on differentiating these network

classes precisely. In some cases it's not the definitions that decide what technology to use,

but rather the technology that indicates what kind of network you have! Since some

protocols are designed for wide area networking, if you are using them, many would say

you have a WAN, even if all the devices in that technology are near each other. On the

other hand, some LAN technologies allow the use of cables that can run for many miles;

most would still consider a mile-long Ethernet fiber link to be a LAN connection, even

though it may span WAN distances.

There are many dimensions in which local and wide area networking technologies differ;

two of the most important are cost and performance. It's easy to establish a high-speed

conduit for data between two systems that are in the same room; much more difficult if the

two are in different states. This means that in the world of WAN, one either pays a lot more

or gets a lot less throughput—and often both.

"Intermediate" Network Types

The blurry line between LAN and WAN is becoming more muddled every years. One

reason is the emergence of “intermediate” network types that straddle the line between

these more familiar terms.

☯ Campus Area Networks (CANs): A campus area network (CAN) is one created to

span multiple buildings in the same location, such as the campus of a university.

Campus area networking is a “gray area”, since neither LANs nor WANs alone are

always well-suited for this type of application. Often, a mix of LAN and WAN

techniques are used for campus networking, depending on the characteristics of the

campus and the needs of the organization.

☯ Metropolitan Area Networks (MANs): Another “intermediate” term that you may see

sometimes is the metropolitan area network or MAN. As the name implies, this refers

to a network that spans a particular small region or a city. Metropolitan area networks

can be considered either as “small WANs” that cover a limited geographical area, or as

“large LANs” that cover an area greater than that normally associated with a local

network. Wireless metropolitan area networks are of course sometimes called

WMANs; IEEE 802.16 is an example of a WMAN standard.

Personal Area Networks (PANs)

Finally, there is one other term occasionally used that should be mentioned: the personal

area network (PAN). This type of network generally means a very small LAN with a range of

only a few feet, intended mostly to connect together devices used by a single person (or

very small group). The term is most commonly used in reference to Bluetooth / IEEE 802.15

wireless technology, so you will sometimes see the terms wireless personal area network

(WPAN) and just PAN used interchangeably.

Key Concept: Networks are often divided by size and general communication

method into three classes. Local area networks (LANs) generally connect together

proximate devices, usually using cables. Wireless LANs (WLANs) are like cabled

LANs but use radio frequency or light technology to connect devices without wires. Wide

area networks (WANs) connect distant devices or LANs to each other. Campus area

networks (CANs) and metropolitan area networks (MANs) fall between LANs and WANs in

terms of overall size; personal area networks (PANs) are like very small LANs and often

appear as wireless PANs (WPANs).

Segments, Networks, Subnetworks and Internetworks

One of the reasons that networks are so powerful is that they can not only be used to

connect computers together, but to connect groups of computers together. Thus, network

connections can exist at multiple levels; one network can be attached to another network,

and that entire whole can be attached to another set of networks, and so on. The ultimate

example of this is, of course, the Internet, a huge collection of networks that have been

interconnected into… dare I say a “Web”? ☺

This means a larger network can be described as consisting of several smaller networks or

even parts of networks that are linked together. Conversely, we can talk about taking

individual networks or network portions and assembling them into larger structures. The

reason why this concept is important is that certain technologies are best explained when

looking at an entire large network at a high level, while others really require that we drill

down to the detailed level of how constituent network pieces work.

Common Terms Describing the Size of Networks

Over time, a collection of terms has evolved in the networking world to describe the relative

sizes of larger and smaller networks. Understanding these different terms is important not

only for helping you comprehend what you read about networks, but also because they are

important concepts in network design. This is particularly true for local area networking,

where decisions regarding how to set up segments and how to connect them to each other

have an important impact on the overall performance and usability of the network. Here are

some of the most common ones.

Network

This is the least specific of the terms mentioned here. Basically, a network can be of pretty

much any size, from two devices to thousands. When networks get very large, however,

and are clearly comprised of smaller networks connected together, they are often no longer

called networks but internetworks, as we will see momentarily. Despite this, it is fairly

common to hear someone refer to something like “Microsoft's corporate network”, which

obviously contains thousands or even tens of thousands of machines.

Subnetwork (Subnet)

A subnetwork is a portion of a network, or a network that is part of a larger internetwork.

This term is also a rather subjective one; subnetworks can in fact be rather large when they

are part of a network that is very large.

The abbreviated term “subnet” can refer generically to a subnetwork, but also has a specific

meaning in the context of TCP/IP addressing.

Segment (Network Segment)

A segment is a small section of a network. In some contexts, a segment is the same as a

subnetwork and the terms are used interchangeably. More often, however, the term

“segment” implies something smaller than a subnetwork. Networks are often designed so

that, for the sake of efficiency, with computers that are related to each other or that are used

by the same groups of people put on the same network segment.

This term is notably problematic because it is routinely used in two different ways,

especially in discussions related to Ethernet. The earliest forms of Ethernet used coaxial

cables, and the coax cable itself was called a “segment”. The segment was shared by all

devices connected to it, and became the collision domain for the network (a phrase

referring generally to a collection of hardware devices where only one can transmit at a

time.)

Each Ethernet physical layer had specific rules about how many devices could be on a

segment, how many segments could be connected together, and so on, depending on what

sort of network interconnection devices were being used. Devices such as hubs and

repeaters were used to extend collision domains by connecting together these segments of

cable into wider networks. Over time, the terms “collision domain” and “segment” started to

be used interchangeably. Thus today a “segment” can refer either to a specific piece of

cable, or to a collection of cables connected electrically that represent a single collision

domain.

Note: As if that potential ambiguity in the use of the word “segment” isn’t bad

enough, it also has another, totally unrelated meaning: it is the name of the

messages sent in the Transmission Control Protocol!

Internetwork (or Internet)

Most often, this refers to a larger networking structure that is formed by connecting together

smaller ones. Again, the term can have either a generic or a specific meaning, depending

on context. In some technologies, an internetwork is just a very large network that has

networks as components. In others, a network is differentiated from an internetwork based

on how the devices are connected together.

An important example of the latter definition is TCP/IP, where a network usually refers to a

collection of machines that are linked at layer two of the OSI Reference Model, using

technologies like Ethernet or Token Ring and interconnection devices such as hubs and

switches. An internetwork is formed when these networks are linked together at layer three,

using routers that pass Internet Protocol datagrams between networks. Naturally, this is

highly simplified, but in studying TCP/IP you should keep this in mind when you encounter

the terms “network” and “internetwork”.

Note: The shorter form of the word internetwork (“internet”) is often avoided by

people who wish to avoid confusion with the proper noun form (“The Internet”).

The latter of course refers only to the well-known global internetwork of computers

and all the services it provides. I personally try to use the word “internetwork” most of the

time in this Guide instead of “internet”, for this very reason.

Key Concept: Several terms are often used to describe the relative sizes of

networks and parts of networks. The most basic term is network itself, which can

refer to most anything, but often means a set of devices connected using an OSI

layer two technology. A subnetwork is a part of a network (or internetwork), as is a segment,

though the latter often has a more specific meaning in certain technologies. An internetwork

refers either generically to a very large network, or specifically to a set of layer-two networks

connected using routers at layer three.

The Internet, Intranets and Extranets

I mentioned in the topic discussing segments, networks, subnetworks and internetworks

that the Internet is really the king of internetworks. After all, you don't get to be called “the”

something unless you pretty much define it.

In fact, the Internet is not just a large internetwork, but substantially more. The Internet is

defined not just as the computers that are connected to each other around the world, but as

the set of services and features that it offers. More than that, the Internet defines a specific

way of doing things, of sharing information and resources between people and companies.

And though it might be a bit melodramatic to say so, to many people the Internet is a way of

life. As Internet use and popularity exploded in the 1990s, many people realized that the

techniques and technologies used on the Internet would be useful if applied to internal

company networks as well. The term intranet was coined to refer to an internal network that

functioned like a “private Internet”. It comes from the prefix “intra”, which means “within”. Of

course, “inter” is the opposite of “intra”, so this makes some people think that an “intranet” is

the opposite of an “internet”. In fact, most intranets are internetworks as well!

As if that weren't bad enough from a jargon standpoint, the buzzword buzzards then

decided to take matters a step further. If an intranet is “extended” to allow access to it not

only strictly from within the organization, but also by people or groups outside the main

company, this is sometimes called an extranet. “Extra” of course, is a prefix that means

“outside” or “beyond”.

So, an extranet is a type of internal, private Internet that, uh, well, isn't entirely internal. An

extranet is an extended intranet, which is really a type of internet that works like the

Internet. (You can start to see why I am not a big fan of these fancy terms. But then, I don't

get to choose them; I just have to help you understand them!) An extranet isn't public and

open to all—it is controlled by a private organization. At the same time, it isn't entirely

private either.

As you can see, the lines between the Internet, intranets and extranets were pretty blurry

from the start, and the concepts are rapidly blending into a diffuse mass of gray, as the

whole computing world becomes more tightly integrated. For example, even if you have an

entirely private intranet, you will want to connect it to the Internet to communicate with the

“outside world” and to allow access to Internet resources. And an extranet may be imple-

mented, in part, through the public Internet infrastructure, using technologies such as virtual

private networking (VPN). I think you get the picture.

The key that binds all of these concepts together is that they all use “Internet technologies”,

a term that is itself somewhat vague. This usually refers to the use of the TCP/IP protocol

suite, which is the defining technology of the Internet, as well as the set of services that are

available on the Internet.

The bottom line is that being told that a company has an “intranet” or an “extranet”—as

opposed to a plain old boring “network”—doesn't tell you much at all. It is best not to rely on

the slogans and instead look at the underlying characteristics of the network or internetwork

itself. Furthermore, when designing such a network, focus on using the technologies and

protocols that make sense—let the marketing people decide what to call it later. ☺

Key Concept: The generic noun internet is a short form for the word internetwork,

while the proper noun Internet refers to the global internetwork of TCP/IP networks

we all know and use. The term intranet refers to an internal network that uses TCP/IP

technologies like the Internet does. An extranet is like an intranet that is extended to

individuals or organizations outside the company. All these terms can be used ambiguously,

so care must be taken in determining exactly what they mean in any given context.

Network Performance Issues and Concepts

Networking is largely about connecting together devices so that information can be shared

between them. Since the idea is to send data from one place to another, a very important

characteristic of any network is its speed: how fast can data be transmitted and received?

This matter of speed turns out to be only one of several issues that determine the overall

performance of a network. The issue is complex enough that I decided a section was

warranted to introduce the various related topics and help you understand how to look at

network performance in a balanced way. When it comes to network performance, there's a

lot of hype out there!

In the computing world, performance is, in general, one of the most often discussed but

least well-understood characteristics of any system or hardware device. This is true of

networking as well. For example, most people know the raw throughput rating of their

network hardware, and may even start to draw conclusions about its capabilities based on

those numbers. Many, however, don't realize that they will never actually achieve that

“rated” amount of performance in the real world.

Most of the other “subtle” issues related to performance are also typically ignored or misun-

derstood, such as the impact of software drivers on hardware performance, and the fact

that certain applications need more than just raw bit speed—they need reliable delivery of

data. But even beyond all of this, one of the most important issues related to network perfor-

mance is understanding what your needs are, to make sure you don't spend too much

money for performance you don't need—or worse, create a network that can't meet your

requirements.

In this section, I discuss various performance issues and concepts that are related to

networking in one way or another. First and foremost, I try to put performance in context,

and also contrast it with non-performance issues. Then, I talk about several key perfor-

mance terms and metrics: speed, bandwidth, throughput and latency. I also talk about some

of the units used to measure network performance. I then explain how the real-world perfor-

mance of a network differs from its theoretical performance, and talk about factors that have

an impact on network performance. I conclude by contrasting full-duplex and half-duplex

network operation, and talking about quality of service, a concept that is especially

important in the use of networks for real-time applications such as streaming multimedia.

Putting Network Performance In Perspective

Performance is probably the “mother of all buzzwords” in the computer industry. There are

many people who consider it the ultimate goal of any computer or computer system, and by

extension, any network. A lot of people spend many dollars and hours of time trying to

maximize it. There's good reason for this: performance is very important. A network that

does not offer adequate performance simply will not get the job done for those that rely on

it. However, it is very important to keep performance in perspective. Successfully

maximizing performance requires that you first take a step back and look at the issue in a

big picture way.

The first question you must ask yourself is also the most essential one: how important is

performance to you? Before you answer this question, recall the old auto racing adage:

“speed costs money—how fast do you want to go?” While there are some situations where

you can get much better performance in a network by spending just a little more money, in

general, you don't get more performance without paying for it in some way. That may mean

more dollar cost for the network, or it may mean a trade-off of some non-performance

characteristic.

If you are designing or specifying a network, it's very important to keep in mind that your

goal is to come up with a system that will meet the needs that were determined for it during

requirements analysis. This means coming up with a network that has a level of perfor-

mance that matches the requirements, and of course leaves some room for expansion.

Unless you have an unlimited budget—and who does?—your objective is not “performance

at any cost”. It is to create a network that meets all of your users' needs, including

balancing performance and non-performance characteristics.

A primary reason why I make a point of trying to keep performance in perspective is that so

many others do not—especially those who are trying to sell you things. Companies are

constantly coming out with the “latest and greatest” high-performance networking technol-

ogies. The usual way that they try to sell them is by attempting to convince everyone that

they just cannot live without this latest advance; that they “need” to upgrade—immediately,

if not sooner! Well, it's simply not the case. As just one example, even though Ethernet

hardware now exists that runs with a theoretical throughput of 10,000 megabits per second,

there are many older networks that continue to work just fine at a hundredth that speed—or

even a thousandth!

Finally, remember that designing a network is usually not an “irreversible”, permanent

decision. Networks can be upgraded and expanded. While it is prudent to build some

“slack” into any network to allow for growth, it is not wise to spend too much on planning for

the future when changes can be made later. This is especially true given that network

hardware prices drop over time. Again here, this is a matter of drawing an appropriate

balance between future performance planning and budget.

Balancing Network Performance with Key Non-Performance

Characteristics

We all know that performance is very important to any network. However, anyone putting

together a network must also be concerned with many different non-performance character-

istics as well. Depending on the network, these can be just as essential to the users of the

network as performance, and possibly even more critical. More than this, non-performance

issues often trade off against performance, and in fact, often more than not one has to be

reduced to get performance to increase.

So, if you want to create a very-high-performance network, you need to understand the key

non-performance network characteristics where you may need to compromise. Here are a

few of these issues, and specifically how they relate to performance concerns:

☯ Design and Implementation Cost: Unless you have bottomless pockets, you need to

be concerned with the network’s costs. As mentioned in the prior topic, cost is the

main trade-off with performance. Going faster costs more money—not always, but

usually.

☯ Quality: The quality of the network is a function of the quality of the components used

and how they are installed. Quality is important because of its impact on all of the

factors described here, such as reliability and ease of administration, as well as perfor-

mance. Quality doesn't trade off directly with performance—you can design high-

quality, high-performance networks—but it does compete with performance for

resources such as budget. All else being equal, it costs a great deal more to

implement a high-quality, high-performance network than a high-quality, low-speed

one.

☯ Standardization: Network protocols and hardware can either be designed to meet

universally-accepted standards, or non-standard, proprietary ones. Standard designs

are almost always preferable, as they make interoperability, upgrading, support and

training easier. Proprietary standards may include enhancements that improve perfor-

mance, but may increase cost and/or make management more difficult.

☯ Reliability: This is related to several other issues, especially quality and performance.

Faster networks aren't necessarily less reliable, but it's more difficult and expensive to

run them as reliably as slower ones.

☯ Expandability and Upgradability: It's very important to always plan for the future

when creating a network. Higher-performance networks can be more difficult to

expand; they are certainly more expensive to expand. Once again, the matter of imple-

menting a network with capacity for future needs now, as opposed to upgrading later if

it becomes necessary, is an important network design decision.

☯ Ease of Administration and Maintenance: Higher-performance networks require

more work and resources to administer and maintain, and are more likely to require

troubleshooting, than slower ones.

☯ Premises and Utility Issues: Implementation of high-speed networks may be limited

by the physical premises, or may have an impact on how they are laid out. Choosing a

higher-speed option may require more infrastructure to be put in place, increasing

cost. The classic example of this is seen in choosing between wired and wireless

options for a home or small office network: with wires you can go much faster, but do

you really want to run the wires?

Anyway, now you have a flavor of how performance balances against some of the other key

issues in networking. The idea of this topic wasn't to convince you not to build a high-perfor-

mance network, just to let you know part of the price you will pay.