nslookup可以解析出IP，仍出現DNS request timed out.

nslookup 可以解析出IP，為何會出現DNS request timed out．timeout was 2 seconds.
nslookup a.b.com.tw
Server: b.b.com.tw
Address: 192.168.1.1

DNS request timed out.
timeout was 2 seconds.
Name: a.b.com.tw
Address: 192.168.1.20

若是以nslookup a.b.com.tw.(多一個"."),即正常解析..無DNS request timed out訊息，請問原因為何？
Server: b.b.com.tw
Address: 192.168.1.1
Name: a.b.com.tw
Address: 192.168.1.20