Why they can view it?

Did you use any sort of security at all when setting up this wlan? Did you just plug the access point in and go? You need to research at least some basic security measures, before setting this up for anyone else. You should at the very least use WEP encryption, preferrably WPA. You should change the defaulut SSID, and disable ssid broadcasting. And if possible I would filter MAC addresses.

Sorry I didn't tell you. The purpose of the wireless router is to serve a conference room, in order to give internet access to guesses in meetings. I tried to used some kind of security measure, but all of them blocked the free access of guesses

changed the ssid from the default and consider not broadcasting it, altho that will take educating the guesses.
see the linksys websites for other tips for securing your wireless network from 'rogue' access.

As far as I understand it - you're in a pretty nasty situation.

The absolute bottom line is that if you want any level of security, you're going to want to encrypt the network. Pick a simple key if you want, but it's going to have to be encrypted.

Changing the SSID and hiding the broadcast is an option, but I personally consider it to be a very poor form of security - any idiot with Ethereal can sit and sniff out the network as soon as someone uses it. Same thing with MAC Filtering - any idiot with Etheral will be able to build up a list of MACs that the router allows and will end up being able to change his NIC's MAC address to let him/her use it. Plus - MAC filtering is even more painful to deal with than a simple password!

Bottom Line: putting up a simple WPA preshared key will offer you the most bang for your effort. If you do SSID hiding, you'll have to tell them what the SSID is *anyway*, so why not just give them a short 6-8 char. password at the beginning of the meeting? You could change it every once in a while and keep the router shutdown when not in use (if you're paranoid like me.) But really, when you get down to it, if your clients support WPA (That usually means Windows 2000 or better - I'm not too sure on the Macintosh side) you should probably just go ahead and go with that.

I would definitely fix the SSID set some security with WEP (or anything). I would also maybe do 1 other thing...

It sounds like this is a small office, so this might not apply. But if the AP is just for guests, you may also want to set ACL so users on the AP can only get to the Internet - no internal resources accessible.

jj