WizNet W5100 not connecting to modern switches and routers

I think your problem is due to the MAC address that is being loading into the W5100. Try setting the 2 low order bits of the first byte to 0.

These bits define whether the MAC address is unicast or multicast and whether it is private or universal. Some switches appear to block ARP requests if these bits are set incorrectly.

I changed the first byte of the MAC address from x'47' to x'44' and things started working.