If you look at the "rbbb_server.ino" file in the link in my previous post, you see that the mac (variable 'mymac') is set to a certain value. You can use any value you like.
Almost every router will be a DHCP server, to assign new IP address to devices that request it. You don't have to do something, it should be enabled in the default settings.
In Windows you can set the ip to a static value (somewhere in the network settings for IPv4) but it is almost always set to 'automatic'. That is DHCP. So you use DHCP every time you turn on the PC.
This example of the Ethercard library,https://github.com/jcw/ethercard/blob/master/examples/testDHCP/testDHCP.ino
shows how to get an IP from the router.
The sketch prints its own IP (it was given by the router DHCP service) to the serial monitor.
I combined both example sketches (some time ago) for a webserver with DHCP and it works.