Make sure you're not encountering a FAT file system bug. This issue bit me, and not sure it's been fixed in all the posted libraries. Large transfers looked corrupt to me, because I was saving to SD card, then re-reading them to process. The bug was saving data larger than 512 bytes, it was not incrementing the sector for the next successive write to finish saving the data; only the first sector was written to the file system correctly.
http://mbed.org/forum/mbed/topic/2273/
I manually fixed my copies of libraries, and then my "networking" code worked.
If this isn't your specific issue with networking, it may be someone else's.
Is there still progress being made fixing the Ethernet libraries for mbed ?
I tried some mixed UDP / TCP apps a few months back and although I had some success they failed in combination and with larger packets / heavier traffic. No one had suggestions and the consensus was that the mbed libraries were flawed and needed work still.
This seems to me to be a big issue but all seems quiet. I'm using another platform currently but keep looking here. Is there progress or am I mistaken in believing there are problems with networking on mbed
K