Split-brain DNS is a configuration where the internal and external DNS environments provide different IP addresses to requests for the same host name, depending on where the request comes from.
If a request for mail.lucernepublishing.com comes from inside the lucernepublising.com network, the address returned might be 192.168.20.10 on the internal network, whereas if a user directly connected to the Internet made the same request to mail.lucernepublishing.com, the IP address returned might be 131.107.43.19.
This configuration is achieved by creating a zone on the internal DNS server for lucernepublishing.com. When a client on the internal network makes a request for mail.lucernepublishing.com, the internal DNS server responds with the IP address for that host, using the A (Address) or CNAME (common name) records that the server maintains for that zone. There is no requirement to forward on the name resolution request to the external DNS servers. However, external
clients who try to contact mail.lucernepublishing.com receive a response from the external DNS server that is authoritative for that zone.