Which device monitors network traffic in a passive manner?

A.
Sniffer

B.
IDS

C.
Firewall

D.
Web browser

Explanation:
A sniffer is another name for a protocol analyzer. A protocol analyzer performs its function in a
passive manner. In other words, computers on the network do not know that their data packets
have been captured.
A Protocol Analyzer is a hardware device or more commonly a software program used to capture
network data communications sent between devices on a network. Capturing packets sent from a
computer system is known as packet sniffing.
Well known software protocol analyzers include Message Analyzer (formerly Network Monitor)
from Microsoft and Wireshark (formerly Ethereal).
A sniffer (packet sniffer) is a tool that intercepts data flowing in a network. If computers are
connected to a local area network that is not filtered or switched, the traffic can be broadcast to all
computers contained in the same segment. This doesn’t generally occur, since computers are
generally told to ignore all the comings and goings of traffic from other computers. However, in the
case of a sniffer, all traffic is shared when the sniffer software commands the Network Interface
Card (NIC) to stop ignoring the traffic. The NIC is put into promiscuous mode, and it reads
communications between computers within a particular segment. This allows the sniffer to seize
everything that is flowing in the network, which can lead to the unauthorized access of sensitive
data. A packet sniffer can take the form of either a hardware or software solution. A sniffer is also
known as a packet analyzer.