I encountered strange setup on a network at company where I do some work. They have internet connection from small ISP located in the same building, all I see is one CAT5 cable with 2 RJ45 connectors. One is going to WAN port on the router and another into LAN port on the same router. This is the only way for internet connection to work. I tried to connect my laptop to one of the RJ45 but it won't work.
Now, what is the purpose for such setup?