The two low bits of a MAC address's first octet carry meaning: bit 0 marks multicast, bit 1 marks locally administered. Generated addresses here set them correctly, so a locally-administered address really is one rather than just looking like it.
What you can control
- Locally-administered addresses have bit 1 set, which is what that range is for.
- Vendor mode uses real OUI prefixes — Apple, Google, Raspberry Pi, VMware, QEMU — with the vendor named.
- Four separator styles including the Cisco three-group dotted form.
- Multicast and broadcast addresses for testing the code paths that treat them differently.
What this is not
Vendor prefixes are real registered OUIs, but the device portion is generated, so no physical adapter carries these addresses.
Questions
What does locally administered mean?
The address was assigned by a network administrator or a hypervisor rather than burned in by a manufacturer. Bit 1 of the first octet marks it, and that range will never collide with a real vendor's.
Are the vendor prefixes real?
The three-octet OUI is genuinely registered to that company. The remaining three octets are generated, so the full address does not identify a device.
Why does Cisco use a different format?
Cisco IOS displays MAC addresses as three dot-separated groups of four hex digits instead of six colon-separated pairs. Same 48 bits, different rendering.