Which company owns MAC address prefix 00:12:3C?

MAC address prefix 00:12:3C is registered to Second Rule LLC in the IEEE Organizationally Unique Identifier (OUI) registry. Any device whose MAC address begins with 00:12:3C has a network interface built by Second Rule LLC.

What is the OUI 00:12:3C?

00:12:3C is an Organizationally Unique Identifier — the first three octets (24 bits) of a MAC address, assigned by the IEEE to Second Rule LLC. It covers every address from 00:12:3C:00:00:00 to 00:12:3C:FF:FF:FF, or 16,777,216 addresses in total. It is also written 00-12-3C.

How do I find the manufacturer of a MAC address?

Take the first three octets of the address — in 00:12:3C:00:00:00 that is 00:12:3C — and look that prefix up in the IEEE OUI registry. If the second-lowest bit of the first octet is set, the address is locally administered (a randomized or private MAC), which belongs to no manufacturer and will not resolve; modern phones and laptops use these by default on each network.

Where is Second Rule LLC located?

Second Rule LLC is listed at 4119 Walnut Street Philadelphia PA US 19104 in the IEEE OUI registry entry for 00:12:3C. This is the address the organization registered with the IEEE, which may differ from its current operating address.

Second Rule LLC

MAC address prefix 00:12:3C is registered to Second Rule LLC in the IEEE OUI registry. Any device whose MAC address begins with 00:12:3C has a network interface built by Second Rule LLC. This prefix (the OUI, or Organizationally Unique Identifier) is assigned by the IEEE and is the first three octets of the address — it covers every MAC from 00:12:3C:00:00:00 to 00:12:3C:FF:FF:FF.

OUI prefix: 00:12:3C
Hyphen form: 00-12-3C
MAC range: 00:12:3C:00:00:00 – 00:12:3C:FF:FF:FF
Addresses in block: 16,777,216
Organization: Second Rule LLC
Address: 4119 Walnut Street Philadelphia PA US 19104
Source: IEEE OUI registry

Have a full MAC address? Look it up— we'll also tell you if it's a randomized (private) address rather than a real manufacturer prefix.

Assignment history (2)

The IEEE registry publishes current state only — this is the change record we've kept since we started tracking it daily.

2016-07-29Assigned to "Second Rule LLC"
archived source →
2026-07-17Assigned to "Second Rule LLC"

Nearby in the registry

The prefixes assigned immediately before and after 00:12:3C:

FAQ

Which company owns MAC address prefix 00:12:3C?: MAC address prefix 00:12:3C is registered to Second Rule LLC in the IEEE Organizationally Unique Identifier (OUI) registry. Any device whose MAC address begins with 00:12:3C has a network interface built by Second Rule LLC.
What is the OUI 00:12:3C?: 00:12:3C is an Organizationally Unique Identifier — the first three octets (24 bits) of a MAC address, assigned by the IEEE to Second Rule LLC. It covers every address from 00:12:3C:00:00:00 to 00:12:3C:FF:FF:FF, or 16,777,216 addresses in total. It is also written 00-12-3C.
How do I find the manufacturer of a MAC address?: Take the first three octets of the address — in 00:12:3C:00:00:00 that is 00:12:3C — and look that prefix up in the IEEE OUI registry. If the second-lowest bit of the first octet is set, the address is locally administered (a randomized or private MAC), which belongs to no manufacturer and will not resolve; modern phones and laptops use these by default on each network.
Where is Second Rule LLC located?: Second Rule LLC is listed at 4119 Walnut Street Philadelphia PA US 19104 in the IEEE OUI registry entry for 00:12:3C. This is the address the organization registered with the IEEE, which may differ from its current operating address.