Port forwarding becomes necessary in some networks due to our inability to traverse symmetric or double NATs. PC World does a great job explaining what this is and some solutions. Before doing anything though, just see if you or the person you're connecting to can enable UPnP. It's much easier than the solution that follows.
We're also going to offer a solution tailored to Parsec below. Unfortunately, this is a complicated process, but once it's done will typically enable connections. If it doesn't work, we recommend setting up a VPN.
As PC World explains, a good way to figure out if you're on a double NAT is to run a traceroute. If that traceroute has multiple hops within the same LAN network, you're stuck behind a double NAT.
The easiest way to solve this is to simply unplug your second network device and connect directly through your primary router. Assuming that's not possible, however, here are some instructions to solve the issue.
The second easiest way is to google and see if you can set up a bridge via the website of your router. You can access that website by following steps 1-6 below. Then follow the steps for creating a network bridge that you find for your router.
COLLECTING INFORMATION ABOUT YOUR NETWORK
First, you need to find out some information about your network and your computer.
On Windows (the Parsec Host)
1. On your Windows computer (the host machine) click Start, and then type cmd into the search field. Press Enter.
2. Type ipconfig /all in the window that pops up. Press Enter.
3. Record the following numbers listed under your network connection(labeled “Local Area Connection” or “Wireless Network Connection”):
- IPv4 Address (or IP Address)
- Default Gateway
FORWARD PORTS VIA ROUTER SETTINGS
5. Now, go to your web browser to access the router settings.
6. Enter your username and password to access your router’s interface. The default router username and password should be available on your router. If it isn't here's a list of common router username and passwords.
Device Username Password
D-Link admin (leave blank)
Netgear admin password
Linksys admin admin
Asus admin admin
DrayTek admin admin
ZyXel admin 1234
TP-Link admin admin
TRENDnet admin admin
Belkin admin (leave blank)
7. To forward ports on your router, look for a tab or menu labeled “Applications & Gaming,” “Advanced,” “Port Forwarding/Port Triggering,” “NAT/QoS,” or something similar.
8. Now, go into the settings and add a rule for Parsec for each IP address you found in the traceroute. Parsec uses 1 UDP port for each connection, so if you just plan on having one friend or yourself connect to the host PC, you can forward ports 8000-8002. If you plan on having more people connect to you all at the same time, you can forward ports 8000-8020. Enable this port forwarding through each IP address for UDP, and hopefully you'll be set. If it doesn't work, you may need to use a VPN.
SETTING UP PARSEC FOR THE CONNECTIONS
Once you've set up port forwarding, visit the networking tab of Parsec's settings.
Once you've forwarded your ports, if you're the server, add the first port to the Server Start Port. We recommend 8000 as a starting port. If you're the guest/client computer, we recommend 9000 as a start port. We also suggest turning of UPnP in case it gets in the way of your port forwarding - this is unlikely, but just in case, you might as well turn it off. You will know that it is saved if the current ports listed under the input field is 8000.