Do some ISP block ports automattically?

I have ran into two sites I have to port forward for example port 6900 to let a client software connect to a video server behind modem and router..

both are using DSL ISP

It's a simple netgear router /port forward/add custom/specifiy your port 6900 /TCP+UDP to lan ip of 192.168.1.23 which is video server lan ip and add it to the list..

If i do http://192.168.1.23:6900 it works fine..from LAN
from WAN http://64.25.xx.xx:6900 nothing...

Firewall is off

I contact the ISP and they said nothing is block from the modem and ISP is wide open..

If i scan the port only port 80 is open

I ended up having to setup Hamanchi VPN to get it to work


A Client with Cable ISP i don't recall having any problem with...Its almost always a client with DSL ISP..

I'm always wonder if these ISP actually block some ports.....then tell u it's not

If an ISP blocks a Port you would not be able to open it on your system since it is blocked before it even arrives to you.

If opening a port on your Router helps, it means that the Application on your computer needs to listen to this port, and thus it needs to be open through the Router (and software Firewall if one is On).

Comcast blocks many ports from client side use. This is how they support their server ban on home service. VPN is the way to get around it. They have relaxed a little and web servers will function. But Mail servers are still a no-no.