Cisco routers are everywhere that networks are. They come in all sizes, from inexpensive units for homes and small offices to equipment costing well over $100,000 and capable of routing at gigabit speeds. A fixture in today's networks, Cisco claims roughly 70% of the router market, producing high-end switches, hubs, and other network hardware. One unifying thread runs through the product line: virtually all of Cisco's products run the Internetwork Operating System, or IOS.

If you work with Cisco routers, it's likely that you deal with Cisco's IOS software--an extremely powerful and...