question about MAC addresses, inter-networking.

I understand MAC really well due to participating in this cisco class, but i was wondering if someone could help me with this.
All MAC addresses with the same OUI must be assigned a unique value (vendor code or serial number) in the last 3 bytes.

so what do most companies use a serial or the vendor code?

Also Macs are referred to as BIA or burned in addresses, considering this does that mean the address is literally engraved on the nic or on the memory inside? If so wouldn't that mean people could tamper with it unless it is ROM. If it is ROM why cant ROM ever be tampered with, more specifically i see why it is called read only memory but what keeps it from being able to be ever altered, erased or anything like that? As well then why is one able to change the MAC through software like windows? i don't understand if it is BIA then why can we change it? does it act as a fake masking or something through the software.?
One last question why can there only be 30 hops to a destination?

> ...what do most companies use a serial or the vendor code?
I don't know.

> ...does that mean the address is literally engraved on the nic or on the memory inside?
It is burned in to a ROM and usually marked in an external way for humans to read.

> If it is ROM why cant ROM ever be tampered with...
Some ROM can be changed. Some ROM have fusible links. I have no idea if NIC vendors use ROM chips with fusible links. But once you blow that link, it is not economical to create an electrical path there again. You could just replace the ROM if you were that desperate.

> ...why is one able to change the MAC through software like windows?
Because sometimes, one needs to do that. Some drivers ignore such changes... so changing the MAC is not always practical via Windows. Some software vendors use a combination of a MAC address and a dongle to prevent the software from being installed on other computers. If the NIC/MAC is a part of a bad motherboard, then you would be down until a new dongle arrives. It could take months for the vendor to get enough orders before they fire up the equipment needed to make a new batch of dongles. So... some software vendors suggest cloning the MAC of the dead motherboard until they can ship you a new dongle.

> i don't understand if it is BIA then why can we change it?
Are you sure that you can change yours?

> does it act as a fake masking or something through the software.?
I'm not positive that I know what you are asking; but if you manage to change it, then yes, the MAC address is fake (if you consider a cloned MAC a fake).

> why can there only be 30 hops to a destination?
30 hops is just the default. It can be set as high a 255 via the '-h" switch.

Let me know it you find a tracert path that resolves hosts past 30 hops. I could not find one for a screenshot for this post.