Arduino-Arduino LAN communication - client not connecting to server

Perhaps we could have more advice about MAC addresses.

From: MAC address - Wikipedia

Bit b2 in the high-order byte should be a 1, and bit b1 should be a zero. It might be possible to recommend the organization identifier in such a way that randomly-chosen MACs are very unlikely, or impossible, to clash with existing devices on the network, like routers, Macs, PCs, etc.