Value of db_link in table dba_db_links

 Hello,
One question:
After having created a new db_link MYLINK between db1 and db2 for the schema apps, the dba_db_links contains the following information:

select OWNER , DB_LINK , USERNAME , HOST from dba_db_links;
APPS           MYLINK.xxx.yyyy          APPS     zzzz

What means the suffix xxx.yyyy, knowing that the instance parameter db_domain is set presently to the value "us.oracle.com"?
Could it be one specified old value of db_domain when creating the database?
Note there's no db_domain specified in file sqlnet.ora.

Thanks in advance.
Armel