For a client accessing sites outside a firewall, VOS should work just fine. However, if you wish to run your own VOS server, but are accessing the internet through a firewall or gateway using network address translation (NAT), you may need to set up a port forward so that external sites are able to contact you.
You should configure your firewall/gateway to forward ports from the external network to the hosts in the internal network on which you wish to run VOS applications. When a VOS site is created, it searches for the first available port starting at port number 4231. Set up your firewall/gateway to forward as many ports as you will need (each application uses one site, which uses one port) starting at 4231 to hosts inside the network. Forward both UDP and TCP, as some applications use TCP and some use UDP. Then, before you run a VOS application on a host inside the network, set a VOS_HOSTNAME environment variable on that to contain your gateway's Internet hostname and the forwarded port number (e.g. example.dyndns.org:4231) See the next section for more information about VOS environment variables.