Internet is
a network of interconnected organisational networks. Communication between two computers on the
Internet is handled by other computers called routers and gateways and is
inherently not private. In contrast with
telephone circuits, Internet communication networks are public and shared. It is simply too expensive to dedicate a line
for the communication and efficiency and cost effectiveness comes at the cost
of privacy.
IP header
of a packet that is sent over the Internet contains the IP address of the
intended receiver in the network. While
surfing the net computers are constantly sending messages to other computers
and remember that most of these computers are parts of local area networks.
Ethernet
is the most frequently used set of protocols for connecting computers together
in local area networks. The way Ethernet
works is that a packet is transmitted to all the computers in the network
and only the computer with the matching IP address is supposed to accept the
packet. Worth pointing out that this is the case if
the network is not so called 'switched'. A
switched network is configured to comprise of a number of segments and a switch
is referred to a filter that forwards the packet from one segment to another.
A computer in the network can be configured to accept all messages
irrespective of IP address of intended receiver (we say the computer is put
in promiscuous mode). A sniffer is software that accepts all the messages
that travel into and out of the computer attached to LAN.
Implications
are pretty high. Note that with Ethernet,
login code and password that is passed from one computer to another is in clear
text format and it is not therefore surprising that sniffing is a very popular
form of attack that is employed by the hacker.