What is Split DNS?

Split DNS refers to using separate internal and external DNS views of your domain's network using internal and external name servers. To set up, configure your internal name servers to forward queries they can't resolve to the external name server. Under Berkeley Internet Name Domain (BIND) 4, use the "forwarders" directive. In BIND 8 systems, use the "forwarders" substatement to configure forwarding. Your external DNS records are configured to contain only a small zone file for your domain, listing things such as Web and FTP server addresses and any translated server addresses you want to publish to the world. Your internal servers hold only the DNS records for your internal networks. When internal users look up host names, the query is answered by internal DNS servers, even if the request is forwarded to an external DNS server for resolution. Internet users who look up host names in your domain are answered by external DNS servers that only know about the publicly accessible resources.
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To know more on this topic, please check the below link.
http://www.isaserver.org/tutorials/You_Need_to_Create_a_Split_DNS.html