Quick summary: Forwarded port works from the outside world, but from the internal network using the external IP the connection is refused.  This is a simplified situation to make the explanation easier:  I have a computer that is running a service on port 12345. This computer has an internal IP 192.168.1.100 and is connected directly to a modem/router which has internal IP 192.168.1.1 and external (public, static) IP 1.2.3.4. (The router is TP-LINK TD-w8960N) I have set up port forwarding (virtual server) at port 12345 to go to port 12345 at 192.168.1.100.  If I run telnet 192.168.1.100 12345 from the same computer everything works. But running telnet 1.2.3.4 12345 says connection refused. If I do this on another computer (on the same internal network, connected to the router) the same thing happens. This would seem like the port forwarding is not working. However...  If I run a online port checking service on my external IP and the service port it says the port is open and I can see the remote server connecting and immediately closing connection. And using another computer that is connected to the internet using a mobile connection I can also use telnet 1.2.3.4 12345 and I get a working connection.  So the port forwarding seems to be working, however using external IP from the internal network doesn't. I have no idea what can be causing this, since another setup very much like this (different router) works for me. I can access a service running on a server from inside the network both through the internal and external IP.    Note: I know I could just use the internal IP inside of the network to access this service. But if I have a laptop that must be able to do this both from inside and outside it would be annoying to constantly switch between 1.2.3.4 and 192.168.1.100 in the software configuration.    Router output:  &gt; iptables -t nat -L -n Chain PREROUTING (policy ACCEPT) target     prot opt source               destination          ACCEPT     all  --  0.0.0.0/0            224.0.0.0/3          DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:25 to:192.168.1.101  DNAT       udp  --  0.0.0.0/0            0.0.0.0/0           udp dpt:25 to:192.168.1.101  DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:110 to:192.168.1.101  DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:12345 to:192.168.1.102  DNAT       udp  --  0.0.0.0/0            192.168.1.1         udp dpt:53 to:217.118.96.203   Chain POSTROUTING (policy ACCEPT) target     prot opt source               destination          MASQUERADE  all  --  192.168.1.0/24       0.0.0.0/0             Chain OUTPUT (policy ACCEPT) target     prot opt source               destination
This is a  about Hairpin NAT (Loopback NAT).   The generic form of this question is:  We have a network with clients, a server, and a NAT Router. There is port forwarding on the router to the server so some of it's services are available externally. We have DNS pointing to the external IP. Local network clients fail to connect, but external work.   Why does this fail? How can I create a unified naming scheme (DNS names which work both locally and externally)?   This question has answeres merged from multiple other questions. They originally referenced FreeBSD, D-Link, Microtik, and other equipment. They're all trying to solve the same problem however.
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