LAN: Networks and Interfaces

Local Area Network (LAN)

The Internet consists of many small-high speed networks,
like those of a company or a university, called LANs.

Computers on a LAN

Computers in a LAN are directly connected by Ethernet cables.
Each machine has an Ethernet card which is referred to
as device eth0 ( if more than one: eth1, eth2, etc ).
These eth devices are called a network interfaces.

Communication on a LAN

Network cards transmit data frames to the LAN
that all other network cards read.
If a card starts to transmit a frame while another card
is transmitting a clash occurs and the card waits
a random amount of time and then tries again.

Each frame has a destination address in its header that
tells what network card it is destined for.
Network cards ignore frames that are not addressed to them.

Network Card Addresses and IP Addresses

Each network card has a physical address of 48 bits called
the hardware address, which is inserted at the time of its
manufacture and has nothing to do with IP addresses.

Each computer on the Internet has a unique IP addresses,
e.g. www.sun.com , in numbers: 64.124.140.181
It consists of a host name and the network it belongs to.

Address Resolution Protocol (ARP):
The destination hardware address must be determined
from the destination IP address before a packet is
sent to a particular machine. This is done is through ARP.

Address Resolution: A machine will transmits a special packet
to asks for the hardware address belonging to an IP address.
The machine with the requested IP address responds.

The transmitting machine stores the result for future reference.
To update this data the ARP protocol has time-outs and re-requests.

Use the command arp to get a list of hardware address to IP mappings.