Network administrators are faced with increasingly sophisticated threats while at the same time having to provide network access for many sources, from highly mobile employees to contractors, vendors, and even customers. In the process, they face perhaps no bigger security challenge than keeping infected or otherwise unsecure laptops, PCs, PDAs, and other devices from accessing their networks. Firewalls limit access from outside the network, and antivirus software blocks malware. However, neither defends against all threats created by unsecure devices connecting to a network.