If you've lost the manual, or acquired a secondhand unit that doesn't have one with it, never fear. Under the Help tab in older versions there are links to the PDF and to the Linksys corporate website. Newer versions have reference documentation built into the firmware, a good thing if your net connection is down.
Unfortunately, you're in trouble if you have to bring in Linksys tech support. On the one occasion that I called them, the first tech I raised couldn't even speak English, and the second was barely competent at it. Both were complete and utter idiots whose response to any nontrivial question was to put me on infinite hold while they went off to query someone else — and then garbled the answer. Judging by their accents, my guess is that Linksys tech support has been outsourced to some particularly benighted corner of the Third World.