Routers.
(a) The Internet is
one of the 20th century's greatest communications developments. It allows
people around the world to send e-mail to one another in a matter of seconds,
and it lets you read, among other things. We're all used to seeing the various
parts of the Internet that come into our homes and offices the Web pages,
e-mail messages and downloaded files that make the Internet a dynamic and
valuable medium. But none of these parts would ever make it to your computer
without a piece of the Internet that you've probably never seen. In fact, most
people have never stood "face to machine" with the technology most
responsible for allowing the Internet to exist at all: the router.
(b) Routers are
specialized computers that send your messages and those of every other Internet
user speeding to their destinations along thousands of pathways.
Types of Routers. The
router can be categorized on the basis of the configuration of the routing
table. The two major types of routers
(a) Static routers.
Static routers require an administrator to manually set up and configuration
the routing table and to specify each route. This is very difficult and proper
record keeping is must for maintenance of the network.
(b) Dynamic routers.
Dynamic routers do an automatic discovery of routes and therefore have a
minimal amount of set up and configuration. They are more sophisticated in that
they examine information from other routers and make packetby- packet decisions
about how to send data across the network. Dynamic router uses
dynamic protocol for
discovery of the neighboring computers and routers.
Keeping The Massages
Moving.
(a) When you send to a
friend on the other side of the country, how does themessage know to end up on
your friend's computer, rather than on one of themillions of other computers in
the world? Much of the work to get a message fromone computer to another is
done by routers, because they're the crucial devices that
let messages flow
between networks, rather than within networks.
(b) Let's look at what
a very simple router might do. Imagine a small company that makes animated 3-D
graphics for local television stations. There are 10 employees of the company,
each with a computer. Four of the employees are animators, while the rest are
in sales, accounting and management. The animators
will need to send lots
of very large files back and forth to one another as they workon projects. To
do this, they'll use a network.
(c) When one animator
sends a file to another, the very large file will use upmost of the network's
capacity, making the network run very slowly for other users.One of the reasons
that a single intensive user can affect the entire network stemsfrom the way
that Ethernet works. Each information packet sent from a computer is
seen by all the other
computers on the local network. Each computer then examinesthe packet and
decides whether it was meant for its address. This keeps the basic plan of the
network simple, but has performance consequences as the size of thenetwork or
level of network activity increases. To keep the animators' work from
interfering with that of the folks in the front office, the company sets up two
separate networks, one for the animators and one for the rest of the company. A
router links the two networks and connects both networks to the Internet
(d) The router is the
only device that sees every message sent by any computer on either of the
company's networks. When an animator sends a huge file to another animator, the
router looks at the recipient's address and keeps the traffic on the animator's
network. When an animator, on the other hand, sends a message to the
bookkeeper asking
about an expense-account check, then the router sees the recipient's address
and forwards the message between the two networks.
(e) One of the tools a
router uses to decide where a packet should go is a configuration table. A
configuration table is a collection of information, including-
(i) Information on
which connections lead to particular groups of
addresses.
(ii) Priorities for
connections to be used.
(iii) Rules for
handling both routine and special cases of traffic.
(f) A configuration
table can be as simple as a half-dozen lines in the smallest routers, but can grow
to massive size and complexity in the very large routers thathandle the bulk of
Internet messages.
(g) A router, then,
has two separate but related jobs-
(i) The router ensures
that information doesn't go where it's not needed. This is crucial for keeping
large volumes of data from clogging the connections of "innocent
bystanders."
(ii) The router makes
sure that information does make it to the intended destination.
(h) In performing
these two jobs, a router is extremely useful in dealing with two separate
computer networks. It joins the two networks, passing information from one to
the other and, in some cases, performing translations of various protocols
between the two networks. It also protects the networks from one another,
preventing the traffic on one from unnecessarily spilling over to the other. As
the number of networks attached to one another grows, the configuration table
for handling traffic among them grows, and the processing power of the router
is increased. Regardless of how many networks are attached, though, the basic
operation and function of the router remains the same. Since the Internet is
one huge network made up of tens of thousands of smaller networks, its use of
routers is an absolute necessity.
Transmitting
Packets.
(a) When you make a
telephone call to someone on the other side of the country, the telephone
system establishes a stable circuit between your telephone and the telephone
you're calling. The circuit might involve a half dozen or more steps through
copper cables, switches, fiber optics, microwaves and satellites, but those
steps are established and remain constant for the duration of the call. This
circuit approach means that the quality of the line between you and the person
you're calling is consistent throughout the call, but a problem with any
portion of the circuit -- maybe a tree falls across one of the lines used, or
there's a power problem with a switch -- brings your call to an early and
abrupt end. When you send an e-mail message with an attachment to the other
side of the country, a very different process is used.
(b) Internet data,
whether in the form of a Web page, a downloaded file or an email message,
travels over a system known as a packet-switching network.
(c) In this system,
the data in a message or file is broken up into packages about 1,500 long. Each
of these packages gets a wrapper that includes information on the sender's
address, the receiver's address, the package's place in the entire message,and
how the receiving computer can be sure that the package arrived intact. Each
data package, called a packet, is then sent off to its destination via the best
available route -- a route that might be taken by all the other packets in the
message or by none of the other packets in the message. This might seem very
complicated compared to the circuit approach used by the telephone system, but
in a network designed for data there are two huge advantages to the
packet-switching plan:-
(i) The network can
balance the load across various pieces of equipment on a
millisecond-by-millisecond basis.
(ii) If there is a
problem with one piece of equipment in the network while a message is being
transferred, packets can be routed around the problem, ensuring the delivery of
the entire message.
(d) The routers that
make up the main part of the Internet can reconfigure the paths that packets
take because they look at the information surrounding the data packet, and they
tell each other about line conditions, such as delays in receiving and sending
data and traffic on various pieces of the network. Not all routers do so
many jobs, however.
Routers come in different sizes. For example :-
(i) If you have
enabled Internet connection sharing between two Windows98-based computers,
you're using one of the computers (the computer with the Internet connection)
as a simple router. In this instance, the router does so little -- simply
looking at data to see whether it's intended for one computer or the other
-that it can operate in the background the system without significantly affecting
the other programs you might be running.
(ii) Slightly larger
routers, the sort used to connect a small office network to the Internet, will
do a bit more. These routers frequently enforce rules concerning security for
the office network (trying to secure the network from certain attacks). They
handle enough traffic that they're generally stand-alone devices rather than
software running on a server.
(iii) The largest
routers, those used to handle data at the major traffic points on the Internet,
handle millions of data packets every second and work to configure the network
most efficiently. These routers are large stand-alone systems that have far
more in common with supercomputers than with your office server.
(e) One of the crucial
tasks for any router is knowing when a packet of information stays on its local
network. For this, it uses a mechanism called a subnet mask.
(f) The subnet mask
looks like an IP address and usually reads "255.255.255.0." This
tells the router that all messages with the sender and receiver having an
address sharing the first three groups of numbers are on the same network, and
shouldn't be sent out to another network. Here's an example: The computer at address 15.57.31.40 sends a
request to the computer at 15.57.31.52. The router, which sees all the packets,
matches the first three groups in the address of both sender and receiver
(15.57.31), and keeps the packet on the local network. (You'll learn more about
how the addresses work in the next section.)
(g) Between the time these
words left the Howstuffworks.com server and the time they showed up on your
monitor, they passed through several routers (it's impossible to know ahead of
time exactly how many "several" might be) that helped them along the
way. It's very similar to the process that gets a postal letter from your
mailbox to the mailbox of a friend, with routers taking the place of the
mail sorters and
handlers along the way. 24. Knowing Where To Send Data Routers are one
of several types of devices that make up the "plumbing" of a computer
network. Hubs, switches and routers all take signals from computers or networks
and pass them along to other computers and networks, but a router is the only
one of these devices that examines each bundle of data as it passes and makes a
decision about exactly where it should go. To make these decisions, routers
must first know about two kinds of information: addresses and
Network structure.
(a) The address has
several pieces, each of which helps the people in the postal service move the
letter along to your house. The ZIP code can speed the process up; but even
without the ZIP code, the card will get to your house as long as your friend
includes your state, city and street address. You can think of this address as
a logical address because it describes a way someone can get a message to you.
This logical address is connected to a physical address that you generally only
see when you're buying or selling a piece of property. The survey plat of the
land and house, with latitude, longitude or section bearings, gives the legal
description, or address, of the property.
(b) Every piece of
equipment that connects to a network, whether an office network or the
Internet, has a physical address. This is an address that's unique to the piece
of equipment that's actually attached to the network cable. For example, if
your desktop computer has a network interface card (NIC) in it, the NIC has a
physical address
permanently stored in a special memory location. This physical address, which
is also called the MAC address ( Media Access Control) has two parts, each 3
(c) The interesting
thing is that your computer can have several logical addresses at the same
time. Of course, you're used to having several "logical addresses"
bring messages to one physical address. Your mailing address, telephone number
(or numbers) and home e-mail address all work to bring messages to you when
you're in your house. They are simply used for different types of messages --
different networks, so to speak.
(d) Logical addresses
for computer networks work in exactly the same way. You may be using the
addressing schemes, or protocols, from several different types of networks
simultaneously. If you're connected to the Internet (and if you're reading this, you probably are), then you
have an address that's part of the TCP/IP network protocol. If you also have a
small network set up to exchange files between several family computers, then
you may also be using the Microsoft NetBEUI protocol. If you connect to your
company's network from home, then your computer may have an address that
follows Novell's IPX/SPX protocol. All of these can coexist on your computer.
Since the driver software that allows your computer to communicate with each
network uses resources like memory and CPU time, you don't want to load
protocols you won't need, but there's no problem with having all the protocols
your work requires running at the same time.
