i second dangermouse, it would be near impossible to stop this type of cheating without impacting the playability on this great site

But dangermouse that can be done: the ones who share the same router (ip) would be known by the admin and noone else would know that and these two dont play in a game together that dont require password,,,but of course these may want to play in a game together and fight with each other, then they set a game and call the ones that they want to play with???

IP limit on public games,

no IP limit on private passworded games.

That could work... And we can make the IPs with multiple users announce it on the boards or tell the Admin. And if one person is clearly cheating, we'd be able to tell (by looking at the game) and take action as necessary. There is a way to check IP, right?

Yes there's a way to check IP.

I don't see why two people who share an IP should be prevented from playing in a public game. Of course, if these people know eachother they should try to prevent that from coming into play in the game. Perhaps, as a courtesy, they should announce the fact at the beginning of the game. On the other hand, as soon as other players know this they will probably assume an alliance between the two when there may not be.

In addition, I sign on from multiple places, including public labs at school. I have no idea who else from my university also logs on here. While the chances are small, it may be that they will end up with the same IP as me at some point. What happens then? Also, when does the IP check occur? Every time the user logs in? Only at the start of the game?

Also, since the subject has come up...I'm friends with three of the other players here: Penfold, Edward and Drunkenlove. It's quite probable we've signed on from the same IP addresses at some point and we've certainly played in public games together. And yes, sometimes we are allies. On the other hand sometimes we are not. And sometimes it doesn't matter a bit as countries are randomly assigned and we never encounter each other.

Do people really do this? :/ I find it hard to believe that people actually create two accounts here to increase their chances, though I can easily believe that certain players tend to ally with players that they've allied with before.

I dont create multiple accounts, but there are those who do it.
To be honest, i dont think these people have the brain power to win at diplomacy anyway, so im not sure its much of a problem here!
Just to say, from my school there are with (user numbers around mine but slightly higher) about 10 players. That could get me one big ban, because our school has one IP!

another example is that I pointed a number of people from my office here, we are all behind a single ip address.

Also if you wanted to cheat like that and the only thing in the way was simple ip limiting you'd just bounce off a proxy or use tor. IP Limits are a waste of time and like most ill thought out security measures do nothing but harm legitmate users.