The RJ203 is an "Ethernet front-end" module that contains an RJ45 jack and 10/100BaseT magnetics (designed to work with the Davicom DM9000B Ethernet controller).

The RJ203's patent-pending design lets you put other components right under it, and thus minimize host PCB size. The front face of the device is translucent. Place status LEDs directly on the host PCB under the RJ203, and you will still see them on the outside.

You can also use the RJ203 together with the EM203 or EM1206 Ethernet modules, which fit right under it. You can order the RJ203 standalone, or in combination with one of these modules.