Archive note: This text comes from the old archive of Nomika Epilekta and is preserved with care for historical and informative reading.

Today, as in some years past, many users and professionals in information technology have encountered a major problem when trying to open a “gateway” to the outside world, the internet.

The reason someone may want to open ports on a computer varies according to what they want to implement. For example, they may want to create a web server, so that web pages are displayed not only locally but also on the world wide web; an internet server, so that we can provide internet to the computers we want under specific rules; a game server, so that we can create a game that people can enter without using other programs, such as Hamachi, for games we have locally; and so on.

Before we proceed to the process we follow in order to open a port, we must understand some basic points in theory. In the following image we see how a modem/router works:

The image above shows a local network that you probably use at home or at work. The computer has received an IP address, internet protocol, from the router, provided of course that the router’s DHCP, dynamic host configuration protocol, gives it one. This is needed so that communication can exist inside the internal network, which may consist of many computers and devices. The router, however, has its own IP, as we see above in the example, 192.168.0.1, and if we use this address we see the router’s configuration panel. From there we are also given an external IP, with which we essentially “go out” to the internet. If we have not made an agreement with our internet provider for a static IP, the external IP changes every two to three days, depending on the provider.

Roughly speaking, we have three IPs: one for the computer, one for the router, internal, and one external. What we must do is instruct the router to open the port or ports we want to use. Usually the problems are the following:
· We do not open the port on the router in the correct way.
· We have not defined a static IP.
· A third-party program is blocking the port.
· We did not define the port in the firewall we may have, so the port opens on the router but not in Windows or in whichever operating system we use.

Let us see in practice how a port is opened through two firewalls and one router. First we need a port-mapping program to see which ports are occupied, closed or open. We can also do this without a program from within Windows.

The command is netstat, and if we type netstat –ano we also see UDP, User Datagram Protocol. If we suppose that we want to open port 9000, or many ports together, or a range of ports, depending on the router we have, we must go to its settings using its IP, as we said above. We open a web browser, such as Internet Explorer, Chrome and so on, and type the router’s IP, in our case 192.168.2.1, where we will be asked for a username and password. At the following link there are all the instructions for opening a port on the specific router, as well as alternative ways if the problem is only on the router and not in some third-party program that we have installed ourselves.

http://www.hol.gr/GR/e-support/topics/Pages/articles.aspx?aid=641&tid=4fce1ca3-9a1e-4ac4-877a-1266274818f9

After we enter the credentials in the router, through the settings we go to create a virtual server, which will allow us to open the range we want. So we go and declare port 9000. In the system we are analyzing now, however, although theoretically we have opened the port, in practice we have opened nothing, because we still have two firewalls to configure.

We start with Windows in the following diagram, to see the steps one by one.

And here we configure the NOD32 firewall, which in turn blocks the ports.

That is it. The ports are now ready for use. If, despite this, our program does not work or the ports do not respond, then we look for problems related either to other programs affecting smooth operation or to the router having a firmware problem. This case is particularly rare if it has not been manually altered, but when dealing with problems, everything is possible.

At the following link you will find another very good site for opening ports and programs. It has solutions for almost all router models and the possible programs that may affect them.

http://portforward.com/english/routers/port_forwarding/Intracom/NetFasteR-IAD/default.htm

John Pap