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An ExpandedNodeId extends the NodeId structure by allowing the NamespaceUri to be explicitly specified instead of using the NamespaceIndex. The NamespaceUri is optional. If it is specified, then the NamespaceIndex inside the NodeId shall be ignored.

The ExpandedNodeId is encoded by first encoding a NodeId as described in 5.2.2.9 and then encoding NamespaceUri as a String.

An instance of an ExpandedNodeId may still use the NamespaceIndex instead of the NamespaceUri. In this case, the NamespaceUri is not encoded in the stream. The presence of the NamespaceUri in the stream is indicated by setting the NamespaceUri flag in the encoding format byte for the NodeId.

If the NamespaceUri is present, then the encoder shall encode the NamespaceIndex as 0 in the stream when the NodeId portion is encoded. The unused NamespaceIndex is included in the stream for consistency.

An ExpandedNodeId may also have a ServerIndex which is encoded as a UInt32 after the NamespaceUri. The ServerIndex flag in the NodeId encoding byte indicates whether the ServerIndex is present in the stream. The ServerIndex is omitted if it is equal to zero.

The ExpandedNodeId encoding has the structure shown in Table 10.

Table 10 – ExpandedNodeId Binary DataEncoding

Name	Data Type	Description
NodeId	NodeId	The NamespaceUri and ServerIndex flags in the NodeId encoding indicate whether those fields are present in the stream.
NamespaceUri	String	Not present if null or Empty.
ServerIndex	UInt32	Not present if 0.