A secure network is only as strong as its weakest link. The high-profile threats most often discussed in the media are external threats, such as Internet worms and DoS attacks. But securing the internal network is just as important as securing the perimeter of a network. The internal network is made up of network endpoints, some of which are shown in the figure. An endpoint, or host, is an individual computer system or device that acts as a network client. Common endpoints are laptops, desktops, servers, smart phones, and tablets. If users are not practicing security with their endpoint devices, no amount of security precautions will guarantee a secure network.

Securing endpoint devices is one of the most challenging jobs of a network administrator, because it involves human nature. A company must have well-documented policies in place and employees must be aware of these rules. Employees need to be trained on proper use of the network. Policies often include the use of antivirus software and host intrusion prevention. More comprehensive endpoint security solutions rely on network access control.

Endpoint security also requires securing Layer 2 devices in the network infrastructure to prevent against Layer 2 attacks such as MAC address spoofing, MAC address table overflow attacks, and LAN storm attacks. This is known as attack mitigation.