I have a simple wireless network in my workplace. We all connect to the internet via a wireless router.
For all those accessing the internet using this wireless n/w, I want to allow access to only a few sites in the office. Is there a way to do so?
It depends on the model of wireless router you have. LinkSys (and some other manufacturers) provide some models of wireless routers that have site control features included, although most of them are about safety (think "NetNanny").
I haven't seen such a model lately, and since the models change so rapidly a suggestion based on a previous model probably wouldn't be very helpful to you anyway. The best bet is to take a look at the product offerings on the manufacturers' web sites or ask them directly. One thing I do recall for certain is that the models that do support these features also come with a higher price tag.
Alternatively, you could set up a Unix or Linux server with the excellent Squid proxy server product, and use it to block/allow certain sites based on their internet domain names. Squid can be set up to work transparently between your router and the internet (if you do this, make sure you also set up a really good firewall like "pf" and be paranoid by blocking everything from the public side [then making additional exceptions only for those protocols you specifically intend to suport] -- this is the internet after all).
Although a Unix/Linux server running Squid might still be a little bit more expensive (unless you already have a decent enough spare computer sitting around that you can use for this -- Unix, Linux, pf, and Squid are all free and open source), it will provide you with a much greater degree of flexibility to block/allow access to more than "only a few sites."
P.S.: It's important to note that HTTPS is generally immune to such blockages since the encryption encapsulates all the HTTP headers as well (including the hostname of the site being requested), so this will only be effective for HTTP. The way to make this work for HTTPS as well is to intercept the SSL certificates and replace them on-the-fly with your own so that you can decrypt, insepct, then encrypt again, but this is likely to cause a lot of unforseen problems (and complaints for users who notice what's going on).
I recommend flashing a compatible router with DD-WRT and configuring the free OpenDNS filtering. Simple easy way to get this done. I prefer the WRT54GL router. http://www.dd-wrt.com/wiki/index.php/OpenDNS http://www.amazon.com/Cisco-Linksys-WRT54GL-Wireless-G-Broadband-Compatible/dp/B000BTL0OA