Sniffing is the process of monitoring and capturing all the packets passing through a given network using sniffing tools. It is a form of “tapping phone wires” and get to know about the conversation. It is also called wiretapping applied to the computer networks.
There is so much possibility that if a set of enterprise switch ports is open, then one of their employees can sniff the whole traffic of the network. Anyone in the same physical location can plug into the network using Ethernet cable or connect wirelessly to that network and sniff the total traffic.
In other words, Sniffing allows you to see all sorts of traffic, both protected and unprotected. In the right conditions and with the right protocols in place, an attacking party may be able to gather information that can be used for further attacks or to cause other issues for the network or system owner
What can be sniffed?
One can sniff the following sensitive information from a network:
Email traffic
FTP passwords
Web traffics
Telnet passwords
Router configuration
Chat sessions
DNS traffic
How it works
A sniffer normally turns the NIC of the system to the promiscuous mode so that it listens to all the data transmitted on its segment.
Promiscuous mode refers to the unique way of Ethernet hardware, in particular, network interface cards (NICs), that allows an NIC to receive all traffic on the network, even if it is not addressed to this NIC. By default, a NIC ignores all traffic that is not addressed to it, which is done by comparing the destination address of the Ethernet packet with the hardware address (a.k.a. MAC) of the device. While this makes perfect sense for networking, non-promiscuous mode makes it difficult to use network monitoring and analysis software for diagnosing connectivity issues or traffic accounting.
A sniffer can continuously monitor all the traffic to a computer through the NIC by decoding the information encapsulated in the data packets.
credit:https://www.tutorialspoint.com/ethical_hacking/ethical_hacking_sniffing.htm