Sqlnet.ora is a text file that provides SQL*Net with basic configuration details like tracing options, default domain, encryption, etc. This file can be found in the ORACLE HOME\NETWORK\ADMIN directory.

It can be used to enable/disable trace.

Reference:

http://www.orafaq.com/wiki/Sqlnet.ora#Sample_sqlnet.ora_files

http://users.handysoft.co.kr/~jelong/net_para.txt

http://www.dba-oracle.com/t_tns_12560_protocol_adapter_error.htm