nios&vhdl

I assume this is a custom board with a CFI flash chip named ext_flash, and a lan91c111 chip.

The 91c111 does not store its MAC address internally; it has to be loaded by the driver during initialization. Altera stores theirs in a part of the flash chip; if you look at the LWIP code, you should see where it goes looking for this during initialization (sorry, I haven't looked at the code myself, so I can't tell you exactly where). You need to either program a new MAC address there, or change the routine to get the MAC address some other way.