Assuming your DSL modem has an Ethernet port as its output (an RJ45 socket) then you can certainly connect it to a switch/hub. You would normally use a crossover cable for this, but it is quite possible that one or both of your devices would auto-switch so even a normal cable might work. I assume from your question that you want to connect several PCs to your internet connection. If so, check that your ISP hasn't limited you to a single MAC address to prevent this
Just to clarify one thing,
If the DSL modem has an RJ-45 jack you would connect it with a straight-through cable to the switch/hub and with a crossover cable to a PC's ethernet card.
Cheers
