MAC Addresses Explained: Formats, OUIs and Locally Administered Bits
Virtual machines, containers and privacy-focused Wi-Fi all use randomly generated MAC addresses. Doing it right means setting two special bits correctly. Here’s the essentials.
Structure of a MAC address
A MAC address is 48 bits, usually written as six hex pairs: 3A:1F:9C:00:B2:7E. Vendor-assigned addresses start with a 24-bit OUI registered to the manufacturer.
The two bits that matter
- Unicast/multicast (bit 0 of the first byte): must be 0 for a normal device address.
- Universal/local (bit 1): set to 1 to mark the address as locally administered, so it can’t clash with real vendor hardware.
RandNow sets both correctly on every address.
Common uses
- Assigning NICs in VirtualBox, VMware, KVM and Hyper-V.
- Testing DHCP reservations and network inventory tools.
- Randomising your own device’s MAC for privacy on public Wi-Fi.
Pro tips
- Choose the Cisco dot format for switch configs.
- Keep a record of MACs assigned to long-lived VMs.
- Use lowercase for Linux tooling consistency.