i've resolved a problem with a mega 2560 and shield W5100.
the ethernet was detected but The problem was  serial port which was returning  adress IP 0.0.0.0  .
The problem was the pins were too long and the contact between mega and shield wasn't correct for the ICSP  connector

So just cut 2 millimeters to all of the shield W5100 pins  to make sure the ICSP connection is well done
between the 2 devices . Just after, an IP adress was available on the serial port monitor .