NET 121b: Essentials of Networking

Chapter 3: Network Adapters


This chapter discusses the OSI network model. The topics of this chapter are:

  1. Understanding the function of a Network Interface Card
  2. Understanding MAC addresses
  3. Where NICs fit on the OSI model

A Network Interface Card (NIC) is the most common device used to directly connect a computer to a network. (It is called a NIC, not a NIC card. The C stands for card.) Sometimes, a motherboard may have this kind of device built into it. Sometimes, users connect by other methods, such as by using modems. A NIC is an example of basic hardware needed by most network devices. Each type of NIC is typically designed for use on one kind of network. For example, you can't use an FDDI card on an Ethernet, or a Token Ring card on FDDI. A driver should come with any card you buy. The card and the driver are specific to the network type, but the other software on a workstation is not. MS Office, for example, does not need to be told which network architecture you are using.

Most NICs have physical addresses burned in at the factory. These addresses are called MAC addresses since they work on the Media Access Control sublayer of the ISO network model. This address serves to provide a unique identifier for every device that has a NIC installed.

When buying a NIC, you should pay attention to three major choice factors:

  • What kind of network does it work on? Check the rated bandwidth of the card as well. If you want an Ethernet card, it is critical to know that your NIC is for an Ethernet, but you also want to know whether it supports 10 Mbps, 100 Mbps, both, or some other standard.
  • What kind of medium is it for? UTP, coax, fiber optic, etc. (Yes, I know, your text has not discussed media yet. Silly book, isn't it?)
  • What kind of bus does it plug into? PCI, SCSI, ISA, etc. A great NIC will do you no good if it is for a kind of expansion slot that your computer does not have.

In order to use a protocol with a Network Interface Card (NIC), the protocol must be bound to the NIC. In ancient times, there was a problem binding more than one protocol to a NIC at the same time. This problem was overcome by two similar technologies. Network Device Interface Specification (NDIS) was created by a Microsoft consortium for this purpose. Open Data-link Interface (ODI) was created by Novell and Apple for the same purpose.

Some NICs use boot PROMs. The idea is this: a workstation has a chip called a boot PROM (Programmable Read Only Memory), that allows the workstation to contact the server, and read an image file that tells it how to boot. The concept is most useful with workstations that do not have hard drives. Why would you have such a workstation? It is typically done to save money, or in places where you do not want the user to save any data to the workstation between restarts.

When installing a NIC or any other expansion bus card, you may need to configure settings for its properties. When installing new equipment, you need to install the hardware itself, you may need to install software (which could include device drivers and application software), and you may need to set whatever system resources the hardware will use. Resource settings can include:

  • IRQ -

    An IRQ is a hardware interrupt request line. There are eight, numbered from 0 through 7 on XT class machines, and sixteen of them numbered 0 through 15 on AT (or better) class machines. Assignment to an IRQ gives a device two things: the right to interrupt the processor and request service now, and a place in the pecking order of such assignments. 0 is always assigned to the system timer, and it has the highest priority. The higher the IRQ number, the lower the priority. Other common assignments are shown below.

    For older devices, never assign the same interrupt number to two devices that could conceivably need attention at the same time. If possible, do not share interrupts at all. (Note that the system has already done this for IRQs 3 and 4.) However, if you are using USB (Universal Serial Bus) or PCI (Personal Computer Interconnect) devices, these devices are meant to share IRQs.

    IRQs (in order of priority) I/O Address Assignment
    0 0040-005F System Timer
    1 0060-006F Keyboard Controller
    2 00A0-00AF Flows to second IRQ controller. IRQs 8 through 15 come from that controller to this line, then go to the CPU.


    0070-007F Real-Time Clock






      SCSI or (Available)


    0238-023F Motherboard Mouse Port


    00F8-00FF Math Coprocessor


    01F0-01F7 Primary IDE drive controller


    0170-017F Secondary IDE drive controller
    3 details --> COM 2 (02F8-02FF) and COM 4 (02E8-02EF)
    4 details --> COM 1 (03F8-03FF) and COM 3 (03E8-03EF)
    5 0278-027F LPT 2 or sound card
    6 03F0-03F7 Floppy drive controller
    7 0378-037F LPT 1


  • DMA channel - Direct Memory Access (DMA) channels give a device direct access to memory. An XT class machine had channels 0 through 3, later machines have 0 through 7, but you will not be able to use channel 4 since it links the upper channels to the lower ones. NT systems DO NOT want you to use this at all.
  • I/O address - I/O Addresses can be compared to a mail drop, a place in memory that the processor will check periodically. This sort of addressing is used for devices that do not need immediate attention from the CPU, whose requests can wait for a cycle or two. Devices should not share addresses, unless confusion and disaster are desired. I/O addresses can also be called ports or sockets.
  • Memory addresses - Essentially, this is RAM for the use of the device, not for communication with the CPU. ROM data is typically copied from the card to the memory address granted to it. A commonly vacant section of memory is the range from CA000 to DFFFF.

You usually install a device first, then install a driver. Most of the time you allow Windows to notice a new device, and allow it to offer a driver for it. However, sometimes the correct procedure is to install a device driver first, which you cannot know unless you read the instructions for installation of the device. This will be a foreign concept to many of you, I am sure. Trust me, there is something to be gained by reading the instructions first.

The text presents an exercise in setting the properties of a NIC in a Windows PC. Regarding the objective of configuring the IP stack on a Windows computer, be aware that you can access the stack from the Network icon in Control Panel (white icon on the near right), or from the Network Connections icon, if you have XP (blue icon on the far right). You can also drill down to the IP stack through Device Manager or My Network Places. As you hear many times, the actual appearance of windows and tabs will vary from one workstation to another, so do not assume that the configuration screens you encounter will look exactly like those in the text.