Arduino Ethernet problem connecting 2 times same ip

Hi everybody,
I have a strange problem with ethernet connection.
Using the basic example code (http://arduino.cc/en/Tutorial/WebClient), arduino make a connection with google.com, and it works without problem.
But if I try to change address, it work only first time. Later I can restart arduino, compile code again,but if fail connection. ("connection failed")
After different test I realized that the problem is related with ip address of destination. Google.com have different IP, so every time I try to connect to google it work because it connect to a different ip address.
Every connection to an address with less ip than google work only the first time. Any other connection fails.
Now I don't understand how is possible, is a router cache problem? a network problem? I really don't know.
Maybe is not strictly related with arduino, I tried to compile the same code with another arduino, and nothing changed. The address connected the first time with arduino will will not work a second time with another Arduino...
Any suggestions?

Here is my example code on the playground. It has all the error checking and fault tolerance stuff that I know of. I've run it without a lockup for days at a time.
http://playground.arduino.cc/Code/WebClient