Freecom NETWORK HARD DRIVE PRO User Manual page 83

Appendix
Someone calling from the internet asks your router for a certain "port". This is the same
GB
as someone calling the receptionist and requesting a certain person by name. Ports are
displayed as numbers but invariably map to pre described services. For instance, when
someone requests a web page, they always ask for the service on port 80, the web server.
Through NAT, it is possible to "map" a certain request for a service (a request for a port)
to a certain PC on the internal network.
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 destination 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 Network Drive Pro, 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.