That assumption creates a false sense of security, and having a false sense of security is worse than having no security at all. Now on to the facts. A true firewall is a protective barrier consisting of both hardware and software run by your ISP to block attacks, while still allowing you to access the Internet freely. Although not as effective as a true firewall, these programs can potentially provide limited protection. Before you go off and download a firewall, however, it's very important that you see through the typically misleading ads and ignorant news stories.
Personal firewalls have many advantages and disadvantages as described below.
It can protect against operating system attacks described in the previous section, but if you regularly update your OS with the latest patches this is unnecessary. It can sometimes help to prevent disconnects due to very slow flood attacks such as from another modem to your modem, but that's like a vest that protects you only against slow bullets - you're still defenseless against real harm. Finally, firewalls can block the action of Trojan horses because a cracker can't remote control you if he can't even connect to you. Trust me, if you don't get smart, a mere firewall isn't going to stop you from getting viruses for the 4th, 5th, or 100th time. Consider if a river is threatening to overflow, you have to try to stop the flood with barriers at the river banks.