The set of Objectsand related information that the Servermakes available to Clientsis referred to as its AddressSpace. The OPC UA AddressSpacerepresents its contents as a set of Nodes connected by References.

Primitive characteristics of Nodes are described by OPC-defined Attributes. Attributesare the only elements of a Serverthat have data values. Data types that define attribute values may be simple or complex.

Nodes in the AddressSpaceare typed according to their use and their meaning. NodeClassesdefine the metadata for the OPC UA AddressSpace. OPC 10000-3defines the OPC UA NodeClasses.

The Base NodeClassdefines Attributescommon to all Nodes, allowing identification, classification and naming. Each NodeClassinherits these Attributesand may additionally define its own Attributes.

To promote interoperability of Clientsand Servers, the OPC UA AddressSpaceis structured hierarchically with the top levels the same for all Servers. Although Nodes in the AddressSpaceare typically accessible via the hierarchy, they may have Referencesto each other, allowing the AddressSpaceto represent an interrelated network of Nodes. The model of the AddressSpaceis defined in OPC 10000-3.

Serversmay subset the AddressSpaceinto Viewsto simplify Clientaccess. Subclause 6.3.4.3describes AddressSpace Viewsin more detail.