with sqlplus i create db link:

create database link xxx.yyy.zzz
connect to uuu identified by ppp
using 'xxx.yyy.zzz';

tnsnames.ora on the server has a nentry :
xxx.yyy.zzz = ....

in sqlplus i select:

select * from table@xxx.yyy.zzz;

ERROR at line 1:
ORA-12154: TNS:could not resolve service name

I can manually connect uuu/ppp@xxx.yyy.zzz;
I can select * from ttt;

So what am I forgeting???????