Fatal NI connect error 12170.

I recently installed an 11 R1 on a Linux box and after creating the database instance started getting the following error a lot in the alert log:

Fatal NI connect error 12170.

VERSION INFORMATION:
TNS for Linux: Version 11.1.0.7.0 - Production
Oracle Bequeath NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
TCP/IP NT Protocol Adapter for Linux: Version 11.1.0.7.0 - Production
Time: 21-JAN-2011 09:26:53
Tracing not turned on.
Tns error struct:
ns main err code: 12535

TNS-12535: TNS:operation timed out
ns secondary err code: 12606
nt main err code: 0
nt secondary err code: 0
nt OS err code: 0
Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=xx.xxx.x.xx)(PORT=xxxx))
WARNING: inbound connection timed out (ORA-3136)

This message is easily corrected by following the instruction in the Oracle document on Metalink:
Net Diagnostic Data Found in 11g Alert Log [ID 745167.1]

The boil down of the solution is to add the following parameter to your sqlnet.ora file to disable a new feature in 11 that is defaulted to ‘ON’:

DIAG_ADR_ENABLED =OFF