What devices do you use? What kind of connection? If you have a high speed connection such as cable or dsl, I would recommend getting a NAT router. The NAT router will take care of the connection with your ISP, masquerade the IP addresses for computers on the LAN, (the NAT function), and provide another layer of defense. The router won't know what to do with a new incoming connection top a port it isn't configured to forward, so it will simply get dropped.
If the RH host is connected directly to the internet instead of a dedicated router, then you will need to do as repo suggested in post #2.
If this is a commercial server on the internetwork and you use a professional grade router such as a cisco router, you can configure the router to do what you want using IOS. This is true for SOHO grade routers as well.