Firewall technology is typically used to filter or block network traffic to and from an internal network. If LDAP requests are coming from outside a perimeter firewall, you need to specify what ports and protocols are allowed to pass through the firewall.
The ports and protocols that you specify depend on your directory architecture. As a general rule, the firewall must be configured to allow TCP and UDP connections on ports 389 and 636.
Host-based firewalls can be installed on the same server that is running Directory Server. The rules for host-based firewalls are similar to the rules for perimeter defense firewalls.