Is this a problem you are having with the router? How are you connecting these 30 machines to the internet? Is it using Airport (wireless).
If I am off track, post a screenshot of the dialogue box at where you are stopped, because I can't actually envisage your problem, hence my guess about it being to do with a router.
Go to System Preferences
Highlight AirPort and click Configure...
Choose “By default, join: Preferred networks”
Select your access point and Remove your access point with the minus button.
Launch your keychain access in Utilities and delete your access point keychain entry.
Go back to the “By default, join:” page and click the plus this time to add your access point. Enter the correct password, save, reboot.
Instead of joining it from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed. Before I went through network diagnostics and all of that stuff in system preferences, and then I tried the "Join Other Network" method, and now it works like a dream.