A couple of years ago I ran an eithernet cable from my router in one >end of the house to a room being converted into a study at the other >end of the house. At the time I allowed a couple of feet of extra cable >but now I'm rearranging the room and need an additional three feet of >cable. Do they make something like a double female eithernet cable >connector that would allow me to connect an additonal cable with the >traditional two male connectors? Thanks.
Just go to an electronics store (aka Jaycar, Dick Smith, etc. here in Australia) and look for an RJ45 female-to-female adaptor. That will couple two RJ45 ethernet leads together. I doubt it complies with any of the ethernet or ISDN standards, but it's grass-roots networking and it works!
I use them regularly when fiddling with network arrangements and I need to extend a cable a short distance to reach a device but have plenty of short ethernet leads on hand.