MAC Address & OUI Formatter

Frequently Asked Questions

What is the OUI in a MAC address?

The first three bytes (24 bits) are the Organizationally Unique Identifier assigned by the IEEE to a manufacturer; the last three identify the specific device.

What do the I/G and U/L bits mean?

The least-significant bit of the first byte is the I/G bit (1 = multicast, 0 = unicast). The next bit is U/L (1 = locally administered, 0 = globally unique/burned-in).

Why are there several MAC formats?

Colon (00:1A:2B…), hyphen, Cisco dotted (001a.2b3c.4d5e), and bare hex all encode the same 48 bits; vendors and tools differ, so normalization helps.

Can this identify the vendor?

It decodes the OUI and bits but does not bundle the IEEE vendor database - look the OUI up in the official IEEE registry for the manufacturer name.

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Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator 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), dot (Cisco 001a.2b3c...), and bare, 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 also decodes the two low bits of the first byte: bit 0 is the I/G bit (0 = unicast, 1 = multicast), bit 1 is the U/L bit (0 = globally unique, 1 = locally administered). Finally it derives the EUI-64 form used for IPv6 SLAAC.

Worked Example

Take 00:1A:2B:3C:4D:5E. First byte 0x00 = binary 00000000: bit 0 is 0 → unicast, bit 1 is 0 → globally unique. OUI = 00:1A:2B, device ID = 3C:4D:5E. EUI-64 derivation: flip the U/L bit of the first byte (0x00 → 0x02), insert FF:FE in the middle → 02:1A:2B:FF:FE:3C:4D:5E. That 64-bit value becomes the host portion of an IPv6 address under stateless autoconfiguration.

Real-World Use

Network engineers use OUI splitting to identify a device's manufacturer from packet captures or switch MAC tables, spot rogue or spoofed adapters, and build DHCP reservations. The locally-administered bit flags randomized MACs (modern phones rotate them for privacy) and virtualization (VMware 00:50:56, hypervisor-assigned NICs). EUI-64 derivation explains why an IPv6 address often embeds FF:FE in the interface identifier.

Common Mistakes

Treating any OUI as a real vendor: a locally-administered or randomized MAC has no meaningful vendor - check the U/L bit first.

Confusing multicast with broadcast: FF:FF:FF:FF:FF:FF is broadcast; 01:00:5E... is IPv4 multicast - both have the I/G bit set.

Forgetting the EUI-64 bit flip: SLAAC inverts bit 1 of the first byte; omitting it produces a wrong IPv6 address.

Format mismatch: pasting Cisco dotted notation into a tool expecting colons silently fails - normalize first.

Limitations & Vendor Lookup

This tool does not bundle a vendor database - resolving an OUI to a company name (e.g. 00:1A:2B → a specific manufacturer) requires the official IEEE registry (the public OUI/MA-L assignment file). Use that registry or a maintained lookup service for vendor identification. Randomized and locally-administered addresses will never appear in the IEEE registry by design. For IPv6 addressing math see the Subnet calculator; this tool covers MAC formatting and bit decoding only.

MAC Address & OUI Formatter

Frequently Asked Questions

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