Understanding the network.pdf - Back to Home

All hosts can see the traffic that passes across a shared media
segment. Normally, they only look at the packets addressed to them.
It is possible to have your computer operate in promiscuous mode by
using a packet sniffer application. A packet sniffer allows you to
examine and capture network traffic that passes across the
transmission medium. If you have a shared-media network, a packet
sniffer is an indispensable tool for diagnosing network problems. It
can also be used illegally to capture information that travels across
the network, such as passwords and credit card numbers.
A variety of sniffer applications are available for the Windows platform.
One of the more popular applications is a package originally called
NetXRay. It is currently sold by Network Associates under the name
Sniffer Pro. On the Macintosh platform, your best bet is EtherPeek, by
the AG Group.
A number of hardware-based dedicated packet sniffers exist on the
market. The Distributed and Expert sniffer product line by Network
General (now also Network Associates) is perhaps the most popular
among the dedicated packet sniffer products. Other hardware-based
manufacturers include Fluke and Hewlett Packard.
Although packet sniffers are very useful, and in some cases essential,
they can be complicated to operate and are expensive. So before
running out and spending a large sum of money on a dedicated sniffer,
first check out the PC-based software tools. This way, you can
familiarize yourself with what you can actually do with a packet sniffer
before making a large commitment of time and money. These tools
have nice GUI front ends and come with some good packet collection
and analysis tools out of the box. In most cases, software-based
sniffers will meet your needs.
One of the essential requirements of a transparent bridge is to deliver
packets between the connected segments completely unaltered. In
other words, this type of bridge is passive, and it operates undetected
by the other connected end-stations on both segments. Because the
bridge, like all the other stations on the network segment, has to wait
until the wire is available for it to forward any packet, it has a memory
buffer that stores the packets it needs to forward. To ensure that no
packet data is lost, the bridge actually copies every packet sent on the
wire into its storage buffer. This process is known as filtering. After
the packet is in the buffer, its destination address is checked against
the bridge's SAT. If the packet's destination address is on the same