Mac Addresses And Address Resolution Protocol; Ethernet Cabling; Cable Quality - Freecom SILVER STORE 2-DRIVE NAS User Manual

FREECOM SILVER STORE 2-DRIVE NAS
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Someone calling from the internet asks your router for a certain "port". This is the same
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 SilverStore, 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 regarding loss and
crosstalk. In addition, there are restrictions on maximum cable length for both 10 and 100
Mbits/second networks.
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