In many cases, you would like to grant external users access to the intranet hosted internally, by enabling NAT/port forwarding through a router or firewall.
However, the intranet requires that all users access the intranet using the same URL (set as the Web Location in the Admin, Setup tab) so it can correctly format dynamic links.
If you use a public domain name for the DNS Zone in your internal network, this is easy to setup. Create a public DNS record which points a subdomain like intranet.mydomain.com to your firewall or router, and then enable NAT to pass these requests on the private IP address of the intranet server.
However, if your internal and external domain names don't match, you have to force your internal DNS server to be authoritative for the FQDN (fully qualified domain name) of your intranet site, so any internal computers resolve intranet.mydomain.com to the servers actual private IP address instead of the public IP address of the router.
To do so, you have to create a new Forward Lookup Zone for the FQDN and create a host record which points to the private IP address.
See the following articles for more information: