For more information on port knocking, visit www.portknocking.org or en.wikipedia.org/wiki/Port_knocking. For configuration detail from one implementation example, check out gentoo-wiki.com/HOWTO_Port_knocking.
An excellent collection of port-knocking software and utilities can be found at www.portknocking.org/view/implementations, and another Windows-based port-knocking server and client can be found at www.security.org.sg/code/portknock1.html.