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

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I can manually connect uuu/ppp@xxx.yyy.zzz;
I can select * from ttt;
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So what am I forgeting???????