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This Built-in DataType is composed of three elements that identify a Node within a Server. They are defined in Table 22.

Table 22 – NodeId Definition

Name	Type	Description
NodeId	structure
namespaceIndex	UInt16	The index for a namespace URI (see 8.2.2).
identifierType	Enum	The format and data type of the identifier (see 8.2.3).
identifier	*	The identifier for a Node in the AddressSpace of an OPC UA Server (see 8.2.4).

See OPC 10000-6 for a description of the encoding of the identifier into OPC UA Messages.

The namespace is a URI that identifies the naming authority responsible for assigning the identifier element of the NodeId. Naming authorities include the local Server, the underlying system, standards bodies and consortia. It is expected that most Nodes will use the URI of the Server or of the underlying system.

Using a namespace URI allows multiple OPC UA Servers attached to the same underlying system to use the same identifier to identify the same Object. This enables Clients that connect to those Servers to recognise Objects that they have in common.

Namespace URIs, like Server names, are identified by numeric values in OPC UA Services to permit more efficient transfer and processing (e.g. table lookups). The numeric values used to identify namespaces correspond to the index into the NamespaceArray. The NamespaceArray is a Variable that is part of the Server Object in the AddressSpace (see OPC 10000-5 for its definition).

The URI for the OPC UA namespace is:

“http://opcfoundation.org/UA/”

Its corresponding index in the namespace table is 0.

The namespace URI is case sensitive.

The IdType element identifies the type of the NodeId, its format and its scope. Its values are defined in Table 23.

Table 23 – IdType Values

Value	Description
NUMERIC_0	Numeric value
STRING_1	String value
GUID_2	Globally Unique Identifier
OPAQUE_3	Namespace specific format

Normally the scope of NodeIds is the Server in which they are defined. For certain types of NodeIds, NodeIds can uniquely identify a Node within a system, or across systems (e.g. GUIDs). System-wide and globally-unique identifiers allow Clients to track Nodes, such as work orders, as they move between OPC UA Servers as they progress through the system.

Opaque identifiers are identifiers that are free-format byte strings that might or might not be human interpretable.

String identifiers are case sensitive. That is, Clients shall consider them case sensitive. Servers are allowed to provide alternative NodeIds (see 5.2.2) and using this mechanism severs can handle NodeIds as case insensitive.

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

IdType	Identifier
NUMERIC_0	0
STRING_1	A null or Empty String (“”)
GUID_2	A Guid initialised with zeros (e.g. 00000000-0000-0000-0000-000000)
OPAQUE_3	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.