What type of adsl modem do you have?
Some have logging inside them that you can read, and some have filtering as well.
It will need an extra piece of hardware, either a dns proxy or an http proxy.
Each request for a webpage needs a dns lookup, so reading the dns logs can give you the basic domain info.
For detailed url info, you'll need an http proxy like squid, and it will need to sit between the adsl modem and the switch.
The easiest overall is a better adsl modem/router with logging, or a managed switch with logging, or build a proxy yourself. Another option is to build a network sniffer, but I suspect you'll want to block certain traffic in the future, so something that is between the switch and the dsl line will be necessary.