I am currently attending a university in which my dorm only has one ethernet jack. However, I have an xbox 360 and my laptop and i want them connected at the same time. The problem is my school has a resnet in which you the student input your MAC address and then they assign your device an ip address so you have to manually set it on your device. My question is would a router allow me to have both my 360 and my laptop connected to the internet at the same time, even though ip addresses are assigned.
If you can ask for multiple IP adresses, get a 2nd IP address using the XBox MAC address. Then set the static IP to the XBox. At this point you can probably test each device individually using your single ethernet jack. Now to be able to use both at the same time, simply get a switch (not a router) and plug them all together and it should work (if the switch has a dedicated "uplink" port, use it for the dorm ethernet cable).
If you can only get a single IP address, you will need a router this time. Get the router's WAN MAC address and request an IP for it. Then configure the router to use STATIC IP from ISP, but leave DHCP for the LAN. After that set both your devices to use DHCP. Connect the WAN port of your router to the dorm ethernet plug and both PC and XBox to the LAN. Note that if you get a wireless router, you will also be able to use wireless devices.