Every network adapter has a MAC address, and on many laptops the address of the built-in Ethernet controller is stored in the BIOS chip. When a board is reflashed with a generic file, that address can be lost or duplicated. This tool converts a MAC address between the formats you meet on labels, in the operating system and in tools, and gives the byte pattern to search for in a dump.
How to use it
- Type the MAC address in any format — from the laptop's label, the operating system or a BIOS screen.
- Read it in colon, dash, plain and dotted formats.
- Use the hex-bytes pattern in a hex editor to find where the address is stored in a BIOS dump.
Formats you will meet
| Where | Format |
|---|---|
| Labels, Linux | 00:1A:2B:3C:4D:5E |
| Windows | 00-1A-2B-3C-4D-5E |
| Some network gear | 001a.2b3c.4d5e |
| Inside a dump | bytes 00 1A 2B 3C 4D 5E |
Some firmware stores the bytes in reverse order, which is why the reversed pattern is also shown.
Checking the address
The first byte carries two flags:
- Multicast bit (bit 0) — set means a group address, never valid for a network card.
- Locally administered bit (bit 1) — set means the address was assigned by software rather than by the manufacturer.
An address of all FF or all 00 means it was never written or was erased — a common result of flashing a generic BIOS on boards that store the MAC in the GbE region. The first three bytes (the OUI) identify the manufacturer who assigned the address.
Worked example: restoring a MAC after reflashing
After a clean BIOS was written to a laptop, its Ethernet adapter shows the address 88:88:88:88:87:88 — a placeholder common in reference files — instead of the address on the label. Searching the original dump for the label's address as bytes finds it in the GbE region. Copying that block into the new file restores the correct MAC. Where these values live and how to carry them over is covered in Serial Number, UUID and MAC Address in BIOS: How to Preserve Them; working with offsets is easier with Hex, Decimal and Binary Converter.
FAQ
Why does my laptop show a different MAC than the label?
The firmware may have been replaced with a file from another board, or the operating system may use a randomised address for Wi-Fi.
Is the Wi-Fi MAC stored in the BIOS?
Usually not; wireless cards store their own. The wired Ethernet MAC is the one commonly found in the BIOS chip.
Can I make up a MAC address?
Use a locally administered address for testing. For a repair, restore the original from the label or the dump.