Starting with SP4, the Domain Name Resolver (DNR) client does not send the last unqualified query to the DNS server, so as to enhance performance.
An unqualified query is a short name DNS host name query, sent after stripping the domain name. If client.domainname.com is unresolved, the unqualified query is devolved to client.com and lastly to client.
If you wish to enable the last short name (client) query, use Regedt32 to navigate to:
On the Edit menu, Add Value name AllowUnqualifiedQuery, a type REG_DWORD value, and set the data to 1. The default is 0.