I tried the example code listed on the ServerConstructor reference page and it failed to compile, giving me:
cannot allocate an object of abstract type 'Server'
It looks like the Server and Client bits have changed names since this document was written, to EthernetServer and EthernetClient. I needed to modify two lines to look like this:
EthernetServer server = EthernetServer(23);
EthernetClient client = server.available();
And I needed to add SPI to the includes:
#include <SPI.h>
Which compiles and uploads just fine. So the page might need an update, or a comment in the example.