The identifier value element is used within the context of the first three elements to identify the Node. Its data type and format is defined by the IdType.

Identifier values of IdType STRING_1 are restricted to 4 096 characters. Identifier values of IdType OPAQUE_3 are restricted to 4 096 bytes.

A null NodeId has special meaning. For example, many services defined in OPC 10000-4 define special behaviour if a null NodeId is passed as a parameter. Each IdType has a set of identifier values that represent a null NodeId. These values are summarised in Table 24.

Table 24 – NodeId Null Values






A null or Empty String (“”)


A Guid initialised with zeros (e.g. 00000000-0000-0000-0000-000000)


A ByteString with Length=0

A null NodeId always has a NamespaceIndex equal to 0.

A Node in the AddressSpace shall not have a null as its NodeId.