Routers And Dial-In-Server

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.