NETWORK/UNREACHABLE
The client could not reach the network endpoint (offline, DNS issues, blocked request, etc.).
Classification
Application code. The wire value and spelling remain the same in every language. A registered code does not establish that its feature is active in every deployment.
Handling
Check the transport and connection state. A timeout or broken response does not prove the server made no change. Reconcile state-changing operations before retrying; reconnect only according to the session policy.