Multiplayer games allow you to play co-operatively with other players around the world. In these cases, one player is "hosting" the game (server), while other players "join" (clients). The game will decide on the best host, so you don't need to worry about that (however, in some cases you might want to force a partcicular computer to act as a host, read below for how to achieve this).
Multiplayer games are have special characteristics. If your computer is the host, the game is pretty much the same as a solo mission. If you're the client, however, you should keep in mind that the quality of your game depends on the quality of your network connection to the host.
The game is really being played on the host, not your computer. Anything you do on your computer must be transferred to the host. This takes time. Roughly speaking, the further away the server is geographically, the longer it will take for your commands to reach it. This creates a delay (or lag) before anything you do takes effect. This lag is known as your ping and is measured in thousandths of a second (milliseconds, or ms).
When you play a network game, your ping is cycled in the network area (top-right of your screen, as you're playing). Pings up to 100 ms are fine, you probably won't even notice the slight delay. If your ping is 100-250 ms, the delay will be noticeable, but you'll quickly get used to it and you'll easily be able to play. Pings above 250 ms will negatively affect your game experience, as you won't be able to react in time.
In high-ping situations, you should focus on the position of the green reticle (which responds instantly to your mouse movements), and not the spaceship itself. The green reticle always shows the final position of the spaceship, even though it might take a while before it actually gets there.
In network games, you might also notice some artifacts as you're playing, like passing through enemies, getting killed by something you didn't touch etc. The worse/slower your network connection is to the server, the more of these glitches you'll experience. Although annoying, these are normal for a network game and there's nothing that can you can do to fix them.
Only use this option to play with computers in your Local Area Network (LAN). All the participating computers need to be on the same subnet. The first computer to select this option will host the game, so if there's a large difference in the hardware specification of your computers, it's recommended that you start your fastest computer first.
Starting with v3.30, you no longer need an internet connection to complete the Network Wizard, so you can now play the game on a LAN where computers aren't connected to the internet. In this case, when the Network Wizard attempts to connect to the internet, wait until it fails (be patient, sometimes it takes a while). Then, simply click on "next" and step through the rest of the wizard.
Depending on your firewall configuration, it's possible that even LAN games are blocked. If you find that your computers can't connect to each other, temporarily disable ALL your firewalls and try again. If the game then works (meaning that your firewalls were indeed to blame for the problem), you can turn your firewalls back on and create a rule for the game to be allowed through the firewall (each firewall does this differently, so consult your firewall's manual for further instructions).
If you are trying to play multiplayer games between computers that are connected wirelessly, be aware that some WiFi routers have a feature called "wireless isolation". It means that multiple computers connected wirelessly to the router won't be able to "see" or communicate one one another. Make sure this option is NOT enabled in your router configuration.
The main option available here is "range". This controls how far away (from your current geographic location) other players can be located. The recommended setting (and, starting with v3.30, the default) is long. However, using this setting you may find that your connection isn't as fast as you'd like (that is, your ping is too high), reduce this setting to medium or low.
Starting with v3.40, an advaned option titled "Ignore ongoing games" has been added. When selected, the matchmaking step will be skipped and you will directly start your own game instead. Since this means you'll be playing alone until someone joins your game, you should normally NOT use this option. It's only recommended in cases where you are repeatedly automatically matched with an unsatisfactory game.
Do NOT use internet multiplayer to play LAN games, because the computers in your LAN won't be able to connect to each other (depending on your network configuration this might actually work, but it normally doesn't).
If you have multiple computers in your LAN that you want to simultaneously play with other computers on the internet (for example, a three-player game where two computers are in your LAN and one is outside), then you must make sure that the network game on the outside computer is completely started before joining it from your LAN computers.
This is essentially the same as an Internet multiplayer game, except that you can choose which people you want to invite. All of your friends (and yourself) should decide on a keyword and then enter that keyword in the relevant box before you start the game. It is not necessary to make the keyword as cryptographically strong as a password. Still, don't keep it too generic, otherwise you might find yourself joining other people's games! The keyword is case-sensitive (that is, "joesgame" is not the same as "JoesGame"), so make sure all players enter it in exactly the same way.
Chicken Invaders includes technology that under normal circumstances allows two computers to communicate even if they're behind firewalls. Depending on the type of your firewall/router and its configuration, however, this can sometimes fail.
When playing multiplayer games, sometimes you may find that you can't connect to games hosted by other people. The master server will report that you're being connected to a game, the world map will appear, but the timer will run out before you can join the game. This is a sign that a router/firewall is blocking communication between the two computers.
It could either be your router/firewall that's causing the problem, the other person's, or even both. Even worse, it could even be a third router somewhere in the path connecting the two computers that lies completely outside your control (this might not be very common, but it's certainly possible).
Troubleshooting these types of problems is very complex, and a solution doesn't always exist. But there are some things you can try (try them in the order listed):