MAC address not retrieved (after 2 years stop)

It'5 days i'm away for work, hopefully tomorrow evening i can come back to my cnc.
Anyeway, to give an answer to tommilight and nwe.
I've 2 phisical lan cards. The 1st (let say A) is the MB's onboard lan card, the 2nd (let say B ) is a separate PCI card.
A is connected directly to mesa card. B is connected to my domestic LAN (then to my router). 2 different phisical cables. This because, if I remember, mesa cards don't love to be connected via router.
Untill 2 years ago, when I worked with my cnc, I used to connect to A (I don t remember if eth0 or eth1), that case my cnc worked perfecly, but i didn't have any access to the web or to my nas.
Then, if I connected to B (in case I need to access the web or my Nas) obviusly the cnc wasn't available.
But I remember I hade 2 different networks (eth0 and eth1).
This year something happened (I didn't do anything but start the pc), so if I connect to the web, i can access internet,but if I connect to mesa (and linux tell me "you're connected to mesa 7i76e) linuxcnc gives me errors.
Now, ok i can try to setup again the network as tommylight suggests, but could it be better to phisically remove the PCI card (B )? I do not need internet or access to my nas whe I use my cnc (that pc is exclusively dedicated  to the cnc machine), I can easily use a normal usb pendrive to transfer the cnc files (I design and program my jobs on another pc),I just care about having my cnc working......