Like Building A House Of Cards - A Tutorial

When a network is established in an office or a home, some of the interactions are as tenuous as a house of cards. There are a number of dependencies between the computers, the router and the protocols that most end users are gracefully sheltered from using a setup CD packaged with the router.

How Does It Work?

Envision your home: A building with a number affixed to the outside to clearly make it a uniquely labeled address. At the front of the house is the door. Here you either grant or deny passage. Once inside the house, there are rooms, each one designated for an individual's space or for a function (e.g., bathroom or kitchen).

A computer network can apply similar descriptions to its parts. The Internet represents the outside world. When someone wants to visit your site, they must connect to your unique address in the world. Once they have found your address, they call at the appropriate port (front door) to request access. If access is granted, then they may try to connect to a computer (a room) within your local area network (LAN) (the house's interior). The computers in your LAN may need to connect to the outside world. In order to do so, they must know where the main exit is from the LAN. This is referred to the gateway. You may compare the gateway to the front door. In fact, the gateway address will be the address of the router on the LAN-side (the inside of the front door).

Who gives out the addresses?

Just as a town has a system for distributing addresses to buildings within its jurisdiction, a Dynamic Host Configuration Protocol (DHCP) Server has its range for distributing unique Internet Protocol (IP) addresses dynamically ("on the fly")to its scope of computers within the LAN. Every computer that has a network interface card (NIC) possesses a unique serial number that it uses as its MAC (media access control) address. The router then tracks, in its memory, the details of all IP addresses given out. [Try not to let the computer acronyms confuse you, I just don't want you to associate MAC to Macintosh. Macintosh, PCs, and Unix machines all have MAC addresses.] The DHCP Server, which can be the router or a server, communicates with a computer when it powers up to help it determine which unique IP address it shall use. Now, we have unique addresses given out to computers on our local area network. We also have unique IP addresses assigned to all computers on the Internet.

How do we identify a computer known as www.tech4now.com?

Stop and consider how most humans do not feel very comfortable in trying to memorize random numbers. In an effort to make life easier for the humans using computers, wise computer scientists developed a system known as the Domain Name System or DNS. DNS is the mnemonic method used to resolve a computer's address based upon its name. Imaging a gigantic table of names and numbers floating around in cyberspace available for each and every computer to reference when it needs to determine what the corresponding address is for each address that someone types into a browser. That is where DNS comes into play. If you were to open a DOS windows (START/Run/"CMD") and type "ping www.tech4now.com", the computer would go out to the Internet (via the Gateway [router]) resolve the address for the computer and advise you of the exact unique address for the computer responding to the name www.tech4now.com. If you checked it today, the value returned would be 206.188.192.76. You could use this value to visit our website. All you need to do is type http://206.188.192.76, it would have the same effect as typing http://www.tech4now.com.

How Does A Computer Get All Of The Addresses?

Remember we discussed DHCP? Well, not only does the DHCP Server give the requesting computer an IP address, it also gives it the information necessary for it to participate in a network--the gateway address (the door to the outside), DNS Servers (how to find the gigantic address resolution table), and the lease term (how long the computer may use its unique address before it must either renew or relinquish the address. Don't worry, most of the time, it keeps the same address.). We are keeping this simple for getting our point across.

Why Are You Explaining This Now?

Last week, one of our clients had issues with their Internet connection--one moment that could surf the web, the next moment, no one could. As a result of a suggestion from the ISP's tech, the client rebooted (powered off and on) their router. After the router had powered back up, the client's computers could, once again, access the Internet. They were very happy until someone needed to print to a network printer that was using a dynamic IP address. Do you remember how I described the router tracking the IP address information in its memory? What do you think happened to that information when the router was restarted? Because the address given to the printer was dynamic (it could change) rather than a static (non-changing) IP address, the printer got a new address from the router. Unfortunately for my clients, all of their computers knew the printing for their computers had to go to the IP address 192.168.100.109 rather than 192.168.100.101. The modest change in the printer's IP address was enough to reek havoc in their network. A quick visit and a modest change to the printer's settings made it a static address to always remember a specific unique address--problem solved. But the thing to remember here is that the printer's address had not been written in stone, but on the wind.

Lesson To Be Learned

James Bond drove a car that had a license plate which rotated to change its numbers and country of origin. By changing its identification, the adversaries were confused and, perhaps, would pursue the wrong vehicle. If you reboot your router and use dynamic addressing for all components of your network, you, too, may confuse the other computers in the network into believing that they are communicating with the right machine while they are not. All critical network resources should be given static IPs or DHCP reservations on the DHCP server to assure there addresses do not change. If you configure your network correctly, it will perform beautifully, if you don't, it's only a matter of time before things begin to fail.

If you found this to be an informative and useful article, please let us know. For support on your network and to make sure you don't lose functionality at the most inopportune moment, call us and we'll get you back on your feet or put you there for the first time. We can be found online at www.tech4now.com, e-mailed at fred@tech4now.com or phoned at (201) 797-5050.