HELP! connect ORACLE7 with TCP/IP by SQL*Plus

I WAS TRYING TO CONNECT ORACLE SERVER WITH SQL*PLUS BY TCP/IP BUT FAILED, THE FOLLOWING COMMANDS WERE ENTERED IN THE SYSTEM PROMPT.

     tcpctl start
     sqlplus user/password _at_T:netid:test

THE ERROR I GOT WAS      ERROR:ORA-06401........ AND I COULDN'T FIND THE ERROR MESSAGE IN RELATED MANUALS. THIS HAPPENED IN  

     NCR/3000 MODEL 3345
     AT/T UNIX SVR 4
     ORACLE 7 engine 7.0.12
     SQL*Plus release 3.1.1.9.1

IF YOU HAVE ANY SOLUTION OR EXPLAINATION ABOUT THIS PROBLEM, PLEASE MAIL TO ME.
I DEEPLY APPRECIATE FOR ANY HELP YOU OFFER. Received on Sat Aug 28 1993 - 10:33:12 CEST