windows server 2008 dns: adding zone for existing external domain

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http://serverfault.com – Our local domain is set up as domainname.local. Our web assets are hosted remotely by a third party (domainname.com). Currently, there is only 1 Forward Lookup Zone on the machine (domainname.local). I need to add an entry for subdomain (sub.dominname.com) that points to a local server ( I tried adding another .com zone and set the nameservers to external DNS ( with an A record pointing the subdomain to, but that failed. Well, technically, the sub resolved correctly, but the rest of the .com stopped working (nslookup failed with a "no answer"). Any ideas fo (HowTos)