| Integer | Clock Class Specification |
|---|
| 0 | Reserved to enable compatibility with future versions. |
| 1—5 | Reserved |
| 6 | The clock is synchronized to a primary time reference. The distributed timescale is PTP. A clock in this class cannot be a slave to another clock in the domain. |
| 7 | The clock has previously been designated as Clock Class 6, but has lost the ability to synchronize to a primary time reference. A clock in this class is in holdover mode and operates within holdover specifications. The distributed timescale is PTP. A clock in this class cannot be a slave to another clock in the domain. |
| 8 | Reserved |
| 9—10 | Reserved to enable compatibility with future versions. |
| 11—12 | Reserved |
| 13 | The clock is synchronized to an application-specific time source. The distributed timescale is ARB. A clock in this class cannot be a slave to another clock in the domain. |
| 14 | The clock has previously been designated as Clock Class 13, but has lost the ability to synchronize to an application-specific time source. A clock in this class is in holdover mode and operates within holdover specifications. The distributed timescale is ARB. A clock in this class cannot be a slave to another clock in the domain. |
| 15—51 | Reserved |
| 52 | The clock is degradation alternative A for a Clock Class 7 clock that is not within holdover specification. A clock in this class cannot be a slave to another clock in the domain. |
| 53—57 | Reserved |
| 58 | The clock is degredation alternative A for a Clock Class 14 clock that is not within holdover specification. A clock in this class cannot be a slave to another clock in the domain. |
| 59—67 | Reserved |
| 68—122 | The clock uses an alternate PTP profile. |
| 123—127 | Reserved |
| 128—132 | Reserved |
| 133—170 | The clock uses an alternate PTP profile. |
| 171—186 | Reserved |
| 187 | The clock is degredation alternative B for a Clock Class 7 clock that is not within holdover specification. A clock in this class can be a slave to another clock in the domain. |
| 188—192 | Reserved |
| 193 | The clock is degredation alternative B for a Clock Class 14 clock that is not within holdover specification. A clock of this class can be a slave to another clock in the domain. |
| 194—215 | Reserved |
| 216—232 | The clock uses an alternate PTP profile. |
| 233—247 | Reserved |
| 248 | The default Clock Class. This class is used if none of the other class definitions apply. |
| 249—250 | Reserved |
| 251 | Reserved for version 1 compatibility. |
| 252—254 | Reserved |
| 255 | The clock is a slave-only clock. |
