You probably won't believe me but
athlon64.domain_not_set.invalid is a valid name for your computer.
This name consists of 3 parts:
- athlon64 : the local host name
- domain_not_set : the domain name
- invalid : the Top Level Domain (TLD) name
If you configure a zone file for "domain_not_set.invalid" for your private nameserver 192.168.1.1 it will even resolve.