A denial of service (DoS) attack occurs when your network receives a very large number of incoming connection attempts that overloads the network's capacity to respond. The volume of traffic forces the computers and routers in the network to operate at or beyond their capacity, which causes them to either flood the network with useless traffic, disrupt connections among machines, or completely disrupt network service. For example, a mail bomb attack might generate tens of thousands of useless email messages.
A DoS attack can make it impossible to do anything else on the network, or it can block legitimate incoming traffic, such as requests for access to a web server or exchange of email. Most DoS attacks are aimed at large businesses, government agencies, and educational institutions, so they're not a common problem for home or small business networks. Even so, it's important to know how to recognize DoS attacks and how to deal with them.
US-CERT, the United States Computer Emergency Readiness Team, has identified three basic types of DoS attack: consumption of limited resources, destruction or alteration of configuration information, and physical destruction or alteration of network components. If your small network suffers a DoS attack, you're likely to see a significant deterioration of network performance—or even a complete breakdown.
If somebody decides to target your network with a DoS attack, you can't do much to discourage them. Therefore, your best protection is to turn on the built-in filters in your firewall and modem, install all the latest security patches to your software and network hardware, and work closely with your Internet service provider at the first sign that your network is under attack. Most ISPs have more sophisticated monitoring and filtering resources that can help them identify and interrupt an attack.
In the United States, DoS attacks are a violation of federal law, so it's appropriate to ask your local FBI field office for assistance in finding and prosecuting the source of an attack.