An organizationally unique identifier (OUI) is the first three bytes of a 6-byte, 48-bit MAC address. The IEEE assigns this portion to a manufacturer, helping MAC addresses remain globally unique. The manufacturer uses the final three bytes to identify individual device ports. section 6.2.2 section 6.6
Reading an OUI from a MAC address
In 0cf5.a452.b101, the OUI is 0cf5.a4, identifying Cisco’s assigned manufacturer prefix. The remaining portion identifies the specific port. Related ports may have sequential addresses such as 0cf5.a452.b102 and 0cf5.a452.b103. section 6.2.2
Use these points when interpreting Ethernet MAC addresses:
- Read the first three bytes, or first six hexadecimal characters, as the OUI. section 6.2.2
- Treat the remaining three bytes as the manufacturer-assigned identifier for the port. section 6.6
- Remember that MAC addresses are normally manufacturer-assigned burned-in addresses, although manual overrides are possible. section 6.2.2
Table 6.1 Decimal numbers and their hexadecimal equivalents (view table figure)
| Dec. |
Hex. |
Dec. |
Hex. |
Dec. |
Hex. |
Dec. |
Hex. |
|||
|---|---|---|---|---|---|---|---|---|---|---|
| 0 |
0 |
8 |
8 |
16 |
10 |
24 |
18 |
|||
| 1 |
1 |
9 |
9 |
17 |
11 |
25 |
19 |
|||
| 2 |
2 |
10 |
A |
18 |
12 |
26 |
1A |
|||
| 3 |
3 |
11 |
B |
19 |
13 |
27 |
1B |
|||
| 4 |
4 |
12 |
C |
20 |
14 |
28 |
1C |
|||
| 5 |
5 |
13 |
D |
21 |
15 |
29 |
1D |
|||
| 6 |
6 |
14 |
E |
22 |
16 |
30 |
1E |
|||
| 7 |
7 |
15 |
F |
23 |
17 |
31 |
1F |
Use the hexadecimal conversion table to interpret the hexadecimal characters used in OUI and MAC-address notation. section 6.2.2
