The decentralized network that all running Fopnu clients form is a pure UDP mesh. There are no central servers or supernodes, and each peer participates in an equal manner.
Your shared file listings are indexed by your own computer and never forwarded to any other peers for indexing. You only process searches against your own files, so your CPU time does not get wasted on lookups that don't involve you.
Because Fopnu uses pure UDP messaging to work, an incoming port is not as important as it is with other P2P apps that use TCP connections. The latest hole-punching technology is built into Fopnu, and even on symmetric NAT connection you should still be able to upload and download very well from most other users.
However, Fopnu will work best if you have an incoming UDP port accessible from the internet.
If you have a broadband home router that connects several computers on your local area network to the internet, simply turning on the UPNP/NAT-PMP option in the Fopnu Settings > Network dialog will usually automatically open a port in the router and nothing else needs to be done. Other routers may have a web-interface that allows you to manually forward the UDP port, which is specified in Fopnu Settings > Network dialog. More information about forwarding ports in general is available on the Tixati support page for broadband routers
If you are using a VPN, this is much the same as using a home broadband router, in that a single public IP is shared by several other members. Depending on the VPN company, you may or may not be able for forward an incoming UDP port, although this is rarely offered. Fopnu will still work very well, and has been designed to compensate for this in several ways.
If you are using a carrier-grade NAT connection such as a cell-phone modem or a connection in a large office, you likely can not map an incoming UDP port. Performance of Fopnu is still likely to be acceptable.
Operating System Firewalls
Most operating systems have a build-in firewall that blocks all incoming TCP connections and UDP messages by default. In most Windows operating systems, you will be prompted when Fopnu starts to decide whether to open a port in the firewall, so there isn't anything you need to do but check off a box and click OK.
On many Linux desktop systems, you will need to check your firewall configuration, which is usually available somewhere in the desktop GUI menus. The exact procedure is different among the various distributions, but it is usually simple to open a UDP port to match the one that is configured into your Fopnu settings.