Thanks to http://www.mirc.co.uk/
Almost all connection problems can be reduced to one of the items in this list. If your particular problem is not in this list read it anyway; this list will give you an idea where to look for a solution to your problem.
NOT ENOUGH BUFFER SPACE AVAILABLE
This means mIRC is having a problem getting a network socket. I have no clue what a socket means on a server, but I do know if your computer can't create one, it can not use your Internet connection to connect to an IRC server. If you are using a lot of other network applications at the same time, you might get this error. Close some other applications and/or reset your Internet connection to fix this problem.
NO ROUTE TO HOST
unable to connect (no route to host)
This normally means you are not properly connected to the Internet. Your Windows can not find out how or where to find the IRC server. It has nothing todo with mIRC or IRC! Check your connection (reconnect if everything seems fine) and try again.
UNABLE TO RESOLVE IRC SERVER
Several issues come to mind here. Either one, your providers' DNS is broken, the server no longer exists under that name, or you typed it's name wrong. This has nothing todo with mIRC or IRC! You better try another server. This one is not going to work today.
CONNECTION TIMED OUT
unable to connect (connection timed out)
Things can get slow on the Internet. Either when you try to reach a far away server or when connections somewhere in between your Internet provider and the IRC server are broken. Very often you didnt have a proper connection to the Internet even before you started mIRC. Patience is usually the best option here. Try it again in a few minutes or try another IRC server. This has nothing todo with mIRC. There is not something you can magically fix.
FORWARD AND REVERSE DNS DO NOT MATCH
*** Your forward and reverse DNS do not match, ignoring hostname.
Closing Link: 188.8.131.52 (You are not authorized to use this server)
The server can not (properly) resolve your IP Number to an IP Name. Your Internet Provider has to fix his DNS system. Until then try another server on the same IRC network.
IT SEEMS THAT YOU DON'T HAVE IDENTD INSTALLED ON YOUR HOST
NO IDENT FOUND
If you get Ident related problems you have not activated the built in Ident server in mIRC, or it is blocked by a firewall or proxy (like Internet Connection Sharing). More and more IRC servers require you to be identified in some way, and they will disconnect you if you are not identified! In mIRC go to File/Options/Connect/Identd/ and enable the Ident server. You can check if you're properly identified by doing a /whois on yourself. The first line in the reply should NOT contain a ~ (tilde) or a - (minus) before your userid. If you have a ~ or a - in it try activating the Ident server built into your IRC client or use a separate Ident server and restart your IRC client and see if it helps.
Closing Link: 0.0.0.0 (Invalid username [ ]) Disconnected
This means the IRC server did not accept the username (User ID) settings in the Ident service (Identd) in mIRC under File/Options/Connect/Identd/. In the error message it shows the part it did not like between the [ ] brackets. Most probably you use characters in your File/Options/Connect/Identd/User ID settings that the server doesn't like. Make sure you remove all strange characters from your User ID. Remove dots, spaces, and all other non-alphabetical characters.
YOU ARE NOT AUTHORIZED TO USE THIS SERVER
Unable to connect - Connection refused.
This means you do not have access to this server. The server has no instructions to let you in (I-line) or it refuses all access for maintnance purposes. Try another server on the same IRC network, preferably a more nearby server, one in your own country or one provided by your own Internet Provider. If you think you should have access to this server contact the administrator of the server and request for an I-Line for the host/IP number that you are trying to connect from.
NO MORE CONNECTIONS ALLOWED IN YOUR CONNECTION CLASS
Connection refused (Max connections at this time)
Closing Link (No more connections)
This means the maximum amount of connections in your class (Y-Line) has been reached. There are too many people from the same company or Internet Provider on this IRC server already. Try another server on the same IRC network, preferably a more nearby server, one in your own country or one provided by your own Internet Provider. If you think you should have access to this server contact the administrator of the server and request for a larger amount of allowed connections for the domain that you are trying to connect from.
TOO MANY CONNECTIONS FROM YOUR HOSTNAME
Closing Link (No more connections)
Closing Link (You are only allowed one connection from your host.)
This means you already have one or more connections to the IRC server and you are not allowed more then the current amount of connections from your address or PC. Normally two connections to the same IRC server are allowed. Check and see if you already have a copy of mIRC running with a connection to this server! Also if you are connecting from behind a firewall or proxy (like Internet Connection Sharing) and thus sharing one IP Number(!), the server will see you all as coming from one address! It will refuse access if more people from your place are on this IRC server already, using the same proxy, or if you have more IRC clients running.
YOU ARE BANNED FROM THIS SERVER
You get this message if you are banned, killed; K-lined. Try another server on the same IRC network, preferably a more nearby server, one in your own country or one provided by your own Internet Provider. Sometimes it is necessary to ban entire domains because abusers connect to the internet via dynamic IP numbers that change and cannot be anticipated. Therefore server admins will ban the entire site/domain, sorry but thats life in a imperfect world. If you feel this is unfair contact the administrator of the server and request for a lift of the K-line on your host, IP number or domain.