The Physical Address

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The Physical Address

As long as the four criteria discussed in the preceding section are met, creating a network is relatively simple. All that is necessary now is some way to distinguish machine A from machine B in a way the network cards can understand. This is done by using a physical address, the unique identifier assigned to a network card. This unique identifier is often referred to as the Mac address, the hardware address, or the ethernet address, but these all represent the same thing. For simplicity, this chapter refers to this identifier as the physical address.

A physical address is a 48-bit address represented by six sections of two hexadecimal values, for example 00-C0-DF-48-6F-13. It is assigned by the manufacturer of the network card before it is shipped to be sold. This identifier is designed to be unique and is often used to help identify a single machine on a network. At this level of the networking model, the Physical layer, data being passed over the network appears to be nothing more than the transmission and error-checking of voltage (1s and 0s) on the wire. These 1s and 0s are transmitted in a certain sequence based on the type of network used. This sequence is referred to as a frame. Within the frame, various pieces of information can be deciphered. The first active component to receive and process the voltage being transmitted onto the network is the network card. Figure 2.4 shows an example of what a standard ethernet frame looks like and the components to which an ethernet card is designed to pay attention.

The network card is responsible for determining whether the voltage is intended for it or some other machine. Each network card is given a set of rules that it must obey. First it listens to the preamble to synchronize itself so it can determine where the data within the frame begins. After it determines where the data begins, it discards both the Preamble and the Frame Check Sequence before continuing to the next process. In the second process, the network card deciphers the data to determine for what physical address the frame is destined. If the destination address matches the physical address of the network card, it continues to process the information and pass the remaining data on for further action. If the destination address specifies some other machine’s physical address, it silently discards the data within the frame and starts listening for other messages.

On a machine running Windows NT 4.0, it is relatively easy to determine its IP address.

Complete the following steps:

1.	From the Start menu, select Programs, Command Prompt.
2.	After the command prompt window appears, type IPCONFIG /all.
3.	Read the information provided by the IPCONFIG utility until you see a section called “Ethernet address.” The value represented is the physical address of the machine.

If a network card discards the preamble and determines that the destination physical address is a broadcast, for example FF-FF-FF-FF-FF-FF, this means the message is intended for all machines connected on that network segment. Whenever a network card receives a broadcast, it assumes the data is relevant and passes the data to the rest of the system for further processing. Network protocols such as NetBEUI use broadcasts to begin communication with a single machine on the network, requiring all machines on the network segment to listen, process the frame, and allow higher layers in the networking model to discard the information. Network protocols such as TCP/IP, although capable of broadcasting, typically determine the specific physical address of the destination machine, eliminating a great deal of broadcast traffic.

Figures 2.5 and 2.6 illustrate the difference between the two types of methods in terms of the processing a machine initiates when receiving a broadcast or directed frame.

In Figure 2.5, each machine on the network opens up the frame and discovers a broadcast address, indicating it must pass the data up to higher layers for processing. In Figure 2.6, only one machine passes the data up to the higher networking layers, while the other machines silently discard the frame as uninteresting data.

It would be unfair to say that TCP/IP does not utilize any broadcasts to communicate, but in general, machines on a network using NetBEUI spend more time deciphering broadcast traffic than machines on a TCP/IP network. This is primarily because NetBEUI is optimized for use on a local area network (LAN), where bandwidth and resources are plenty. NetBEUI is also enormously easy to install and configure and requires almost no ongoing intervention on behalf of the user. It’s only significant weakness is that it is not a routeable protocol, meaning that it has no addressing characteristics that allow packets to be moved from one logical network to another.

TCP/IP, on the other hand, is designed for wide area network (WAN) environments where routers are the common connection method between two locations. Because of its routability and almost surgical (precise and efficient) use of bandwidth resources, it is clearly the favorite for this type of environment. However, it does require significantly more knowledge and experience on the user’s part to install and configure it correctly before it can be utilized. This is probably why Microsoft deems it necessary to test user’s and administrator’s knowledge of this protocol (that is, they don’t have a test dedicated to NetBEUI or NWLink).

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