Which of the following protects a network with Internet connectivity from attackers on the Internet?

A.
A router

B.
A gateway

C.
A bridge

D.
A firewall

Explanation:
A firewall prevents attackers on the Internet from direct access to LAN computers. It is often referred to as the first line of defense for a network with Internet connectivity.
Incorrect Answers:
A: A router connects multiple networks together to form an internetwork. It does not provide protection against internet based attackers.
B: A gateway is a hardware and software combination that connects two dissimilar wired network environments. It does not provide protection against Internet based attackers.
C: A bridge connects two similar wired network segments together. It does not provide protection against internet based attackers.
References:
Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 11, 16-18.
David Groth and Toby Skandier, Network+ Study Guide (4th Edition), Sybex, Alameda CA, 2005, pp. 33, 36-37.