There are two usual causes for game networking errors: Firewalls and routers.
The first thing to try is to turn off all your firewalls. If that fixes the problem, turn the firewalls back on and add appropriate exceptions. If your firewall works on a per-application basis, you just need to allow the Mayhem Intergalactic program full access to the internet. If you're having problems with hosting the game, you may also need to allow incoming connections on the port that you're hosting on (the default port is 7465, but you can change it to any port on the "create server" screen).
If you've turned off all your firewalls and people still can't join the game that you're hosting, and especially if you have more than one computer in your house, then you probably have a router issue. Most home routers are
Network Address Translation (NAT) routers, which means they share one public IP address between multiple computers. Each computer shares that one
public IP address, but has its own
private IP address.
Computers on the internet only know your public IP address, and all information directed to any computers in your house is sent to that public IP address. So your router needs to know how to sort that information and deliver it to the private IP address (this is the step that puts the "NAT" in "NAT router").
For normal web browsing, email, etc., the router is smart enough to figure it out on its own. But if you're hosting a server (for example, a Mayhem Intergalactic game server) then you need to
tell your router that you're doing that, so that the router knows where to send incoming connections.
Telling the router that you're hosting a server is called
"port forwarding". It's a little complicated to set up, but you only have to do it once.
Different routers have different methods for forwarding ports. The first step is to find out your router's brand and model number. This will usually be written on the side of the router, and on the packaging that came with it. For example, my router has "D-Link DI-704P" on the side. D-Link is the brand, DI-704P is the model number. Yours will probably be different.
A router physically looks like a little box, and has all of your computers' network cables plugged into it. (Or, if you use wireless networking, it might have an antenna instead.) It will typically also have another network cable plugged into your
modem, which your ISP will have supplied (or required you to buy). Occasionally the modem and router are integrated into one unit.
Once you've found your router's model number, visit
this page at PortForward.com and search the page for it. Click on your model number, and then go to the "Default Guide". You'll get a detailed guide, specific to your router, on how to set up port forwarding. When it asks for a port, put in
7465, unless you changed that number on the "Create game" screen. You should forward
both TCP and UDP.
I know this is a total pain. I'll try to get some dedicated servers up so that you don't have to host games yourself.