Verify TCP/IP connectivity if you are using TCP/IP:
Can the server be reached using the ping utility?
If the tnsnames.ora file uses logical service names resolved by a DNS server, verify that the DNS server is properly resolving the name by using the logical name in the ping.
If the logical name is not resolved by a DNS server, verify that the logical name is in the hosts file on the client.
If the logical name cannot be resolved, try using the server’s IP address in tnsnames.ora.
Verify IPX/SPX connectivity if you are using IPX/SPX or Novell networking:
Can you perform a NetWare login to a Novell server on the network?
Verify named pipes connectivity if you are using named pipes in a Windows 95/98/NT environment:
Using the Network Neighborhood icon, can you see other nodes on the local network?
Verify SQL*Net operation on the client: