The Counter64 type represents a non-negative integer which monotonically increases until it reaches a maximum value of 2^64-1 (18446744073709551615 decimal), when it wraps around and starts increasing again from zero.
Counters have no defined "initial" value, and thus, a single value of a Counter has (in general) no information content. Discontinuities in the monotonically increasing value normally occur at re- initialization of the management system, and at other times as specified in the description of an object-type using this ASN.1 type. If such other times can occur, for example, the creation of an object instance at times other than re-initialization, then a corresponding object should be defined with a SYNTAX clause value of TimeStamp (a textual convention defined in [3]) indicating the time of the last discontinuity.
The value of the MAX-ACCESS clause for objects with a SYNTAX clause value of Counter64 is either "read-only" or "accessible-for-notify".
A requirement on "standard" MIB modules is that the Counter64 type may be used only if the information being modeled would wrap in less than one hour if the Counter32 type was used instead.
A DEFVAL clause is not allowed for objects with a SYNTAX clause value of Counter64.