Appropriate network communication software must be installed and running before you run the database server. If you are running reliable network software with just one network installed, this should be straightforward.
For more information about network communication issues, see Client/Server Communications.
You should confirm that other software requiring network communications is working properly before running the database server.
If you are running under the TCP/IP protocol, you may want to confirm that ping and Telnet are working properly. The ping and Telnet applications are provided with many TCP/IP protocol stacks.