Denial of Service Attack

Firewalls offer internet security from many different kinds of attacks. One kind you may have heard in the news is a "denial of service" attack. This type of attack is extremely difficult to counter. What happens is that the hacker sends a request to computer asking to connect to it. When the server responds with an acknowledgement and tries to establish a session, it cannot find the system that made the request. By inundating a computer with these unanswerable session requests, a hacker causes it to slow to a crawl or eventually crash.