Guid 0

[Tally]

Having done a bit of quick research (really quick), I have found out that the problem is the COD2 Activision Master Servers:

What the Master Server does is converts CD Keys to GUID Numbers FOR cod2 servers you play on and ensures your CD Key is valid and is not already in use.

Outages and Problems

When it breaks, some or all of the Master's functions become unavailable. This causes a NUMBER of problems:

Can't Refresh Server List

If you've ever clicked on the "Refresh" button while viewing the list of servers and had it just sit there and hang... Then you are well acquainted with this annoyance. While the "Refresh" button will not work when the Master is down, the "Quick Refresh" will always work because it does not request a fresh list of servers; it merely updates the status of all the servers currently in your list. Note: if you've already hit "Refresh" and get no response, it is too late to use "Quick Refresh")

Can't Ban People

When the GUID Number look-up process breaks, then the CoD2 server will not be able to associate a ban against the CD Key of the offending player. When the Master server is unavailable, this causes players who connect to receive a GUID number of 0. GUID 0 is a very commonly asked question. Players that have GUID 0 can't be banned. The banclient command is ignored and the tempbanclient command will kick them from the server, but won't actually temporarily ban them. The person you are trying to remove will still be able to immediately rejoin as long as the Master server is down.

The Banned Come Back

People that have previously been successfully banned or vote-kicked can immediately re-join the server.

Since the method the server relies on to look up CD Keys is broken, all of those banned users (CD Keys) in your ban list.... Don't do a damn thing while the Master server is down.
Banned CD Keys will actually work

People that install pirated versions of the game are using a stolen CD key code that has been permanently banned by Activision. When that look-up process is broken, All of the chumps without a valid CD Key get to play online.
The Hackers Come Out

Since the pirate crowd is only allowed to play on all the CoD2 servers while the Master server is down, Master server outages bring in tons of players that are normally prevented from playing.

A disproportionate number of these new players seem to be hackers. Since they are normally already banned from all the servers to begin with, they have little to risk, hence: there seems to be less of deterrent against hacking among this class of players. Especially when they figure out that they are essentially unbannable while the Master is down.

All of the above is not mine. It was taken from this website:

http://cod2.smaert.com/wookie/index...._Master_Server