When you say private server, I assume you mean cracked?
COD2 Call of Duty 2 Cracked Servers & Private Servers - EPCGaming.com
COD4 : https://lmgtfy.com/?q=cod4+cracked+servers
WAW: https://lmgtfy.com/?q=world+at+war+cracked+servers
BO: You can download the SKIDROW version and play offline using BOLoader
MW2: Alteriwnet. If you are banned try the XUID Spoofer: https://www.mpgh.net/forum/269-call-d...-9-2011-a.html