MAC Address & OUI Formatter

Frequently Asked Questions

What is an OUI and who assigns them?

An OUI is a 24-bit prefix the IEEE assigns to hardware manufacturers, allowing each to issue up to 16 million unique MAC addresses. The IEEE maintains a public OUI registry at standards.ieee.org.

Why does my phone show a random MAC on Wi-Fi?

iOS, Android, and Windows generate a random locally-administered MAC per network to prevent tracking. The U/L bit (bit 1 of the first byte) will be 1, confirming it is locally administered.

What is EUI-64 and why does it flip a bit?

EUI-64 is a 64-bit IPv6 interface identifier formed by inserting FF:FE in the MAC's middle and flipping the U/L bit, converting IEEE convention (0=global) to IETF convention (1=global).

Can I look up the vendor in this tool?

No. This tool formats and analyzes the MAC structure but does not bundle a vendor database. Use the IEEE public OUI registry or a third-party lookup service for vendor identification.

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How It Works

A MAC address is 48 bits (6 bytes / 12 hex digits). This tool strips any separator, validates the length, then re-emits every common format: colon (00:1A:2B...), hyphen (Windows style), Cisco dot (001a.2b3c...), and bare hex, in upper and lower case.

It splits the first 3 bytes as the OUI (Organizationally Unique Identifier - the vendor block assigned by the IEEE) from the last 3 bytes (the device-specific NIC ID). It decodes the two low bits of the first byte: bit 0 is the I/G bit (0 = unicast, 1 = multicast/group), bit 1 is the U/L bit (0 = globally unique, 1 = locally administered or randomized). Finally it derives the EUI-64 form: insert FF:FE in the middle of the MAC and flip the U/L bit of the first byte.

Use Cases

Network engineers use this tool to:

Identify device manufacturers from packet captures, switch MAC tables, or DHCP logs by reading the OUI.

Spot randomized or spoofed MAC addresses by checking the locally-administered bit before querying a vendor database.

Build DHCP reservations or firewall rules that need the address in a specific format (colon, hyphen, or Cisco dot).

Derive the EUI-64 interface identifier for IPv6 SLAAC to predict what link-local address a device will self-assign.

Distinguish IPv4 multicast group addresses from broadcast in packet analysis (01:00:5E... vs FF:FF:FF:FF:FF:FF).

Tips

Check the U/L bit before doing a vendor lookup - if bit 1 of the first byte is 1, the MAC is locally administered (randomized or VM-assigned) and will not appear in any IEEE OUI database.

Modern smartphones rotate MACs for privacy - iOS 14+, Android 10+, and Windows 10+ all use randomized MACs by default when probing for networks; the OUI will not point to Apple, Google, or Qualcomm in those cases.

VMware uses 00:50:56, 00:0C:29, and 00:05:69 for its hypervisor-assigned NICs; these are globally unique OUIs assigned to VMware specifically for virtual machines.

The EUI-64 bit flip is easy to forget - SLAAC inverts the U/L bit (bit 1 of the first octet) when building the IPv6 interface identifier; a globally unique MAC (bit=0) becomes 1 in EUI-64.

Normalize format before pasting into tools - Cisco CLI expects the dotted format; Linux ip/ifconfig uses colon; Windows arp uses hyphen; mismatches silently fail.

FAQ

What is an OUI and who assigns them?

An OUI (Organizationally Unique Identifier) is a 24-bit prefix that the IEEE assigns to hardware manufacturers. Each OUI block allows the manufacturer to assign up to 16 million unique MAC addresses to their devices. The IEEE maintains a public registry of all OUI assignments at standards.ieee.org/develop/regauth/oui.

Why does my phone show a random MAC on Wi-Fi?

To prevent tracking across networks, iOS, Android, and Windows generate a random locally-administered MAC for each Wi-Fi network they probe or join. The U/L bit (bit 1 of the first byte) will be 1, identifying it as locally administered rather than a vendor-burned global address.

What is EUI-64 and why does it flip a bit?

EUI-64 is a 64-bit interface identifier used in IPv6 SLAAC. It is formed by inserting FF:FE in the middle of the 48-bit MAC and flipping the U/L bit. The bit flip converts the IEEE convention (0 = globally unique) to the IETF convention used in IPv6 (1 = globally unique interface ID).

Can I look up the vendor from an OUI in this tool?

No. This tool formats and analyzes the MAC structure but does not bundle a vendor database. The IEEE publishes the complete OUI-to-manufacturer mapping at standards.ieee.org/develop/regauth/oui/public.html. Third-party lookup services also maintain this data.

MAC Address & OUI Formatter

Frequently Asked Questions

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