Configuring the Oracle Network Environment
Name resolution and the listener
A client connection needs an address (host, port, protocol) and a service name. Listener configuration lives in listener.ora on the database server; client-side name resolution lives in tnsnames.ora (local naming) or is derived at connect time by Easy Connect (host:port/service). sqlnet.ora sets the naming-methods order, authentication options and encryption.
Dynamic versus static registration
With dynamic registration, PMON registers the instance's service names (SERVICE_NAMES, defaulting to DB_NAME.DB_DOMAIN) with the listener named by LOCAL_LISTENER (default port 1521 on the local host). Registration happens as the instance starts, so lsnrctl status shows the service with a state such as READY once the instance is open.
With static registration, you hard-code a SID in SID_LIST_LISTENER inside listener.ora. Use it when the listener must accept connections before the instance can register itself: database creation with DBCA, RMAN duplicate, and Data Guard broker configurations.
lsnrctl START|STOP|STATUS|SERVICES|RELOAD manages the listener; ALTER SYSTEM REGISTER forces dynamic registration.
Traps: tnsping proves only that the name resolves and the listener is reachable - not that the database is open or the credentials are valid. A service shown as UNKNOWN means the listener is running but no instance has registered, not that the server is down. Local sqlplus / as sysdba uses a bequeath connection and does not need the listener at all.
Rule of thumb: connection string → listener address → registered service → instance. If tnsping succeeds and SQL*Plus fails, look at registration, SERVICE_NAMES and authentication, not at the network.