Start at the fifth "Sticky Post" in this forum and have a look at FNET. You may need to search Sourceforge for fnet as clicking on the links in the posts didn't work for me today.
Another thing to check out is uTasker (http://www.utasker.com/). Mark posts tto this forum frequently. I don't think it directly supports the MCF51 series, but the Ethernet drivers may help get you started.
Tom