The following is the first installment of a multi-part series on the fundamentals of routing. Each tip is excerpted from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.
 |
|
About the book
|
|
Routing First-Step explains the basics of Internet routing in language all of us can understand. This book takes you on a guided tour of routing, starting with systems you are familiar with: the postal system, the telephone system, and the interstate highway system. From there, you'll learn routing simply and easily. Whether you are looking to take your first step into a career in networking or are interested only in gaining knowledge of the technology, this book is for you!
Author William R. Parkhurst, Ph.D., CCIE, manages the CCIE Development group at Cisco Systems. The CCIE Development group is responsible for all new CCIE written qualification and laboratory exams. Prior to joining the CCIE team, Bill was a Consulting Systems Engineer supporting the Sprint Operation. He first became associated with Cisco Systems while a Professor of Electrical and Computer Engineering at Wichita State University. In conjunction with Cisco Systems, WSU established the first CCIE Preparation Laboratory.
|
|
|
Internet addressing
To begin our discussion on computer communication over a network, this section
looks at the similarities between mail delivery between houses and data delivery
between computers. The endpoints in mail delivery are houses, and the endpoints
between electronic data delivery are computers.
Certainly there can be other endpoints
in both systems. Letters can be delivered from a house to a business, from a
business to a house, between two businesses, and so on. Electronic data delivery
can be from a news service to your cell phone or personal data assistant (PDA),
from your computer to your friend's pager, from environmental sensors in a building
to the heating and cooling control systems for that building, and so on. But to
keep the discussion simple, it will suffice to concentrate on mail delivery between
houses, and electronic data delivery between computers. The first analogy is that
an endpoint in a mail delivery system, a house, is equivalent to the endpoint in a
computer communication system, a PC. (See Figure 3-1.)
Figure 3-1 - Equivalent Endpoints in the Mail and Data Communication Systems
In the mail delivery system, the function of the post office is to deliver mail to a
particular house. In the computer communication system, the function of the
Internet is to deliver data to a particular PC. Yet, in both systems, the endpoint is
not the ultimate destination. For mail, the ultimate recipient is a person. For data,
the ultimate recipient is an application such as an e-mail program, a web browser,
an audio or video program, an instant messaging program, or any number of wonderful
applications that exist today. (See Figure 3-2.)
Figure 3-2 - Final Destinations in the Postal and Electronic Data Delivery Systems
Although the ultimate recipient is a person or a software application, the responsibility
of the systems stops when the mail, or data, is delivered to the proper house,
or computer. However, as part of the address, you still need the ultimate recipient;
either a person or an application, even though this information is not used for
delivery to an endpoint. The endpoint uses the name or application to enable
delivery to the recipient.
Because the two systems are analogous, it is instructive to revisit the format of an
address in the mail delivery system and see if you can use a similar format for
electronic data delivery:
Name
Street Number, Street Name
City, State
Although there are five distinct pieces of information in the mail address (name,
street number, street name, city, and state), you can consider an address to contain
only four pieces of information. For endpoint delivery, you can ignore the name
field. You are left with
Street Number
Street Name
City
State
The postal system routers (core, distribution, and access) use the state, city, and
street names to deliver the mail from the source access post office to the destination
access post office. The street number is not needed until the mail arrives at the
access post office that is directly connected to the destination street. So, the
address can be broken down into
State, City, Street Name
and
Street Number
The state, city, and street name information enables the mail to get close to the
destination (a particular street). The street number is used to deliver the mail to the
proper house. What is the analogy in the computer world to houses on a street?
Recall from Chapter 1 that a group of computers can directly communicate with
each other through a switch residing on a local-area network (LAN). So a LAN is
the computer equivalent to a street. (See Figure 3-3.)
Figure 3-3 - LAN of Computers Is Similar to a Street of Houses
Chapter 1 also mentioned that computers have an address, and the most common
technology used for computer communication is Ethernet. The sample Ethernet
address that was presented in Chapter 1 was 00-03-47-92-9C-6F.
Before you learn more about Ethernet addresses, take the following quiz to make
sure you understand the concepts described so far:
1. What number base is used to represent the Ethernet address?
Answer: Hexadecimal, because the symbols C and F are not used in the
other number bases that we discussed. Computers compute using binary.
The hexadecimal representation is for our benefit because it is easier to read
and write.
2. How many bytes are in an Ethernet address?
Answer: Six. One hexadecimal digit contains 4 bits, or 1/2 bytes. Two
hexadecimal digits contain 8 bits, or 1 byte. An Ethernet address contains 12
hexadecimal digits or 6 bytes.
3. How many bits are in an Ethernet Address?
Answer: 48 (8 bits per byte).
4. How many Ethernet addresses are possible?
Answer: 248 or 281 trillion, 474 billion, 976 million, 710 thousand, 6 hundred
fifty-six (281,474,976,710,656).
An Ethernet address is not a property of your PC. An Ethernet address is a property
of the Ethernet card, or built in Ethernet port in your PC. If you put a new
Ethernet card in your PC, the Ethernet address of your PC changes.
By itself, an Ethernet address cannot deliver data between two endpoints on the
Internet. The reason is that there is no structure to an Ethernet address. There are
many manufacturers of Ethernet cards for computers, and each manufacturer is
assigned a block of Ethernet address to use for their particular brand of card.
An analogy would be to have 281,474,976,710,656 postal addresses that are sold
in a local postal address store. Each local postal address store is given a block of
numbers from the total range of numbers that are possible. A postal address is just
a number between 0 and 281,474,976,710,655. When you build a house, you
would go to the local postal address store and your house would be assigned one
of the numbers that hasn't yet been assigned. Everyone in your city would need to
get a number assigned from the local postal address store. Because people will not
be going to the store in any order, numbers will be assigned randomly throughout
the city. The only way that these numbers can be used to deliver mail is if every
post office at every level (core, distribution, and access) maintained a list of every
number, and the route to reach that number. Therefore, every post office would
need to maintain a list of 281,474,976,710,656 addresses and the route to get
there. Obviously, this is not scalable. So in addition to an Ethernet address, you
need another address that has a structure analogous to the structure of the postal
address. What you need is an Internet addressing protocol.
All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.
