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Appendix
MAC Addresses and Address Resolution Protocol
An IP address alone cannot be used to deliver data from one LAN device to another.
To send data between LAN devices, you must convert the IP address of the destination
device to its media access control (MAC) address. Each device on an Ethernet network
has a unique MAC address, which is a 48-bit number assigned to each device by the
manufacturer. The technique that associates the IP address with a MAC address is known
as address resolution. Internet Protocol uses the Address Resolution Protocol (ARP) to
resolve MAC addresses. If a device sends data to another station on the network and the
destination MAC address is not yet recorded, ARP is used. An ARP request is broadcast
onto the network. All stations on the network receive and read the request. The desti-
nation IP address for the chosen station is included as part of the message so that only
the station with this IP address responds to the ARP request. All other stations discard
the request.
The station with the correct IP address responds with its own MAC address directly to
the sending device. The receiving station provides the transmitting station with the
required destination MAC address. The IP address data and MAC address data for each
station are held in an ARP table. The next time data is sent, the address can be obtained
from the address information in the table.
Ethernet Cabling
There are two different types of cabling for Ethernet networks. Originally they used
thick or thin coaxial cable, but most installations currently use unshielded twisted
pair (UTP) cabling. The UTP cable contains eight conductors, arranged in four twisted
pairs, and terminated with an RJ45 type connector. There are 2 types of RJ-45 cables
available today, the Media Dependent Interface (MDI) cable and the Media Dependent
Interface - Crossover (MDI-X) cable. The first cable is used to connect routers to PCs and
the second to connect PCs directly to PCs. The sent and receive wires are switched from
beginning to end in the crossover cables. Some Ethernet switch products, such as the
MusicPal, are able to sense the polarity of a connection and automatically adapt to the
proper cabling type.
Cable Quality
A twisted pair Ethernet network operating at 10 Mbits/second (10BASE-T) will often
tolerate low quality cables, but at 100 Mbits/second (10BASE-Tx) the cable must be rated
as Category 5, or "Cat 5", by the Electronic Industry Association (EIA). This rating will be
printed on the cable jacket. A Category 5 cable will meet specified requirements regard-
ing loss and crosstalk. In addition, there are restrictions on maximum cable length for
both 10 and 100 Mbits/second networks.