Thanks for the info !
It doesn't help me to fix my problem, but I understand now why I get such values.
I always use the full method with all the parameters, so normaly all should be good.
I will try to put a sniffer on my network ... maybe I'll get some other interesting info. Because normally the Arduino should receive something from my box ! But in the sketch I can print only data at the upper sofware level. I suppose there a other software level between the "client.read()" and the lower level (where the ethernet signal is received). Or do you know whether it's possible to activate traces into the Arduino ?