Since I changed my ISP, I have had issues logging into sites (e.g. StackExchange), MMORPGs and connecting to SMTP servers. I did some research and it turns out that my public IP address is changing constantly (in a matter of minutes at most). I get my private IP address via DHCP but so far it's using the same IP address consistently.
On StackExchange I get the error message that my client is behaving suspiciously and therefore login is not possible. In MMORPGs I get a message while loading the game that the IP address has changed since logging in. In Thunderbird emails are downloaded from the server but cannot be sent since the connected to the SMTP server is not possible. I have tested it with two different SMTP servers.
Is there a way to remedy this? As far as I know just using NAT shouldn't cause any issues. Is the rapid change of the public IP address what actually leads to the response from the SMTP to be lost on the way back to my client? Would a WLAN router be any help at all?
Tested on Win7 and Linux Mint 16.
Thank you in advance for any advice and pointers in the right direction.