What are CID and IAB assignments?

They are two different assignments, and neither behaves like an ordinary vendor prefix. A CID, Company ID, is a 24-bit identifier that cannot be used to generate globally unique MAC addresses, because the addresses derived from it are locally administered. An IAB, Individual Address Block, contained 4,096 EUI-48 addresses sharing a 36-bit prefix. The IEEE stopped issuing IAB assignments on 1 January 2014 and replaced them with MA-S.

CID: a 24-bit identifier that cannot make a unique MAC address

A CID has the same length as an OUI and looks the same in the registry, but the two are not interchangeable. The IEEE states that a CID is not used to create globally unique EUI-48 or EUI-64 identifiers. Addresses an organization derives from its CID have the U/L bit set, which marks them as locally administered: they belong to no globally administered block, and a lookup that tests the address against registration data finds no registrant for the interface itself.

That is why a CID is the wrong answer to "I need a range of addresses my devices can present worldwide". It is the right answer when an organization needs a company identifier for purposes other than manufacturing network interfaces.

IAB: the small block that MA-S replaced

An IAB was a block of 4,096 EUI-48 addresses, issued to organizations that needed far fewer addresses than the 16,777,216 of an MA-L. The IEEE retired the scheme on 1 January 2014 and points new applicants to MA-S, which also provides 4,096 EUI-48 addresses. The block sizes sets out the three current lengths.

Why a lookup still answers for them

An IAB issued before 2014 is still a real assignment, and the interfaces built from it are still in use, so the registration data is kept and the lookup resolves those prefixes. A CID is likewise a real registration. In both cases the answer names the organization the identifier was assigned to, and the blockType field of an API response says which kind of assignment matched — the API documentation lists the values.

An existing IAB assignment provides globally unique EUI-48 addresses and remains usable until its exhaustion. A CID does not provide a globally unique MAC address block. A device presenting an address derived from a CID is locally administered, and a lookup reports that as no vendor found unless the address happens to fall inside a registered block.

Look one up

Paste a prefix into the MAC address lookup to see which assignment type it matches and which organization holds it.