1
I have 3 computers in my network connected to the same router.
Computer A - my desktop computer Computer B - Web server Computer C - Mail server and SQL server
Everything is set up correctly externally which means I can telnet to IMAP port (143) on Computer C and Web server port (80) on Computer B.
But suddenly, in like a 10 minute interval or more often I cannot telnet to Computer C from the outside. I can telnet from Computer A and B. And I can telnet from Computer A to B from the outside.
I do not think the problem is the router. Computer C is new with Windows 2012 and I guess it is either a network problem, configuration etc on that computer. Do you have any more ideas of how to test this or?
2It is possible your ISP is blocking your mail server when it detects it. – Scott Goodgame – 2013-06-29T12:18:58.220
No, I have mail server on Server B. And it works with same ports. Thanks anyway! – Henrik – 2013-06-29T14:06:06.243
Got windows update recently? Some windows update may change your firewall rules... (on computer C). – Darius – 2013-06-29T16:07:35.010
Thanks for your feedback - found the reason - the switch got the same ip as the computer C. – Henrik – 2013-07-01T07:35:56.267