In all lecture halls and in many seminar rooms you can't connect to TUGnet without authentication. You have to use VPN or PPPoE. The same goes for "open access" network sockets at students' computer practice rooms (but here only PPPoE works).
When you try to use VPN you have to use a valid IP address to connect to our VPN concentrator (so use DHCP!), for PPPoE the IP address is unimportant.
You need a "TUGRAZonline-Account", a "network access password" and PPPoE client software installed. If you are using Windows XP or Mac OSX such client software is installed already. If you want to use PPPoE with Windows 95, 98, NT or 2000 client software has to be installed by you. Using Linux you can use rp-pppoe for example, but there are other possibilities, too.