Something is probably messed up with either your tnsnames.ora or sqlnet.ora on the server. Create the database link with the full description from your tnsnames.ora like:
This should eliminate your networking issues. If this works, you have a problem with your tnsnames.ora or sqlnet.ora.
create database link xxx.yyy.zzz
connect to uuu identified by ppp
using '(DESCRIPTION =(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = jeffhpc)(PORT = 1521)))CONNECT_DATA =(SERVICE_NAME = nt817.us)))';
If this doesn't work, you may have a tcp/ip issue...