So each instance of Fedora will run in a virtual machine (VM) defined by configuration in VirtualBox on the host. By default, the VMs' NICs will be NAT-attached. For NAT-attached VM NIC's, VirtualBox running on the host acts like a router. It provides a DHCP server and NAT translation. This is, in effect, like the common arrangement of a private LAN connected via router to an ISP.
It has the same restrictions; services running on computers on the (virtual) private LAN can only be reached through the router by port forwarding. And it has more restrictions (from VirtualBox 2.0.6 documentation) that are seldom significant but are included here for completeness:
There are four limitations of NAT mode which users should be aware of:
ICMP protocol is very limited: Some frequently used network debugging tools (e.g. ping) rely on sending/receiving messages based on the ICMP protocol. The ICMP protocol cannot be used directly by normal applications such as VirtualBox, as it would, at least on Linux hosts, require root permissions (more precisely CAP_NET_RAW). Since this is not desirable, no attempt has been made to support ICMP to addresses other than 10.0.2.2 and 10.0.2.15. If you try to ping any other IP address you will not get any response.
Receiving of UDP broadcasts is not reliable: The guest does not reliably receive broadcasts, since, in order to save resources, it only listens for a certain amount of time after the guest has sent UDP data on a particular port. As a consequence, NetBios name resolution based on broadcasts is not always working (but WINS always works). As a workaround, you can use the numeric IP of the desired server in the \\server\share notation.
Protocols such as GRE are unsupported: Protocols other than TCP and UDP are not supported. This means some VPN products (e.g. PPTP from Microsoft) cannot be used. There are other VPN products which use simply TCP and UDP.
Forwarding host ports < 1024 impossible: On Unix-based hosts (e.g. Linux, Solaris, MacOS X) it is not possible to bind to ports below 1024 from applications that are not run by root. Therefore if you try to configure such a port forwarding, then the VM will refuse to start.
What network connectivity do you want? From Internet (or real LAN) to your VM(s)? From applications running on your host OS to your VM(s)? Only between the VMs?