ExpandedNodeId values shall be encoded as a JSON string using the format defined in 5.1.12. The NodeId portion of the ExpandedNodeId uses the rules from 5.4.2.10.ExpandedNodeIds with a ServerIndex of 0 are encoded using the <node-id> form. All other ExpandedNodeIds use the <server-uri> form unless the first abnormal state occurs as described below.

The first abnormal state occurs when the encoder cannot map a ServerIndex to a ServerUri. In this case, the encoder shall encode the ServerIndex using the <server-index> form. The decoder shall pass this ServerIndex to the application.

The second abnormal state occurs when the decoder cannot convert a ServerUri to a ServerIndex. If this occurs the decoder shall set the ServerIndex to 0, the NamespaceIndex to 0, the IdType to String and the Identifier to the JSON string.

When the ServerIndex is 0, decoders shall replace a NamespaceUri with a NamespaceIndex when a mapping exists or if a mapping can be created. If no mapping exists or one cannot be created the NamespaceUri is stored in the NamespaceUri field of the ExpandedNodeId.