What is the function of forward lookup zone (DNS)?

Forward lookup zone is use to resolve domain name into IP address.

Forward lookup zones support the primary function of Domain Name System (DNS), that is, the resolution of host names to IP addresses

If your question isThe Domain Name System (DNS)? it is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities.

1. This resource record creates a mapping in the reverse lookup zone that typically corresponds to a named host (A) resource record for the DNS computer name of a host in its forward lookup zone. The in-addr.arpa domain applies to all TCP/IP networks that are based on Internet Protocol version4 (IPv4) addressing.

It used external ISP IP address to find local IP address