This is a common scenario. It is not a question of routing, it is a question of name resolution.
Whether you host your own domains or use a third party, DNS records would need to resolve the domain name to an IP address.
A common approach would be - an A record (also known as a host record) links a domain to the physical IP address of a computer hosting that domain's services.
A CNAME (Alias) record points to an A (Host) record. You can create multiple CNAME records and point them to an A record. The most common CNAME records are the subdomains www and ftp.
With this approach, you can host several services on one machine, using a single IP address.
If the ftp servers resided on separate devices sporting RFC1918 addresses, a reverse proxy could act as a gateway and take requests from the Internet and forwarding them to servers on the internal network.