What is to open a port on the router, and how to test if it is really open?

Sometimes we need to open the door for some application, program, or even games so that we can access resources on the internet, or even speed up downloads, in short, there are many purposes in opening a door.

But what exactly is "opening a door"? This is what we will explain in this tutorial.

The internet is a network where everyone can communicate, transfer data, through an IP address.

When using the internet using a router, it is possible to share the connection with several computers.

Every device that accesses the Internet must have 1 valid IP address on the Internet, that is, without a router, like the ones we know, every computer would have to have an Internet connection.

This is because the router identifies itself as a computer, and it does the translation of these packets so that they are identified by only 1 IP address which is valid on the Internet.

This feature is known as NAT (Network Address Translation) or slightly translating: "Network address translation".

The router has the function of creating a network, also called a subnet, and it connects to another network, the Internet.

The IP addresses from within the subnet are not visible on the Internet, that is, the person responsible for taking the packet from the local network and transferring it to the Internet is the router.

To exemplify the context, follow the example:

Local Network: 192.168.0.0/24 (or 192.168.0.0 and mask 255.255.255.0).

Internet: 177.178.20.30/32 (or 177.178.20.30 and mask 255.255.255.255).

Every time a computer communicates with another on the same network, the router forwards it to the local computer. Every time the IP address is outside the network, it is transferred to the Internet, as if the router was asking for certain information.

1. Computer on the local network 192.168.0.3 requested www.google.com

2. Router received the request from www.google.com from the IP Computer: 192.168.0.3

3. Router with public IP 177.178.20.30 sends the request to Google from the page at www.google.com

4. Google receives 177.178.20.30 asking for www.google.com

5. Google sends the page www.google.com to 177.178.20.30

Router with NAT feature, has a table where it stores the information that came out. Then you know that the order for www.google.com came from 192.168.0.3 and sends.

6. Router sends www.google.com to 192.168.0.3.

Of course, there are many other steps in the middle of this path, such as ACK, SYN, connection to DNS via UDP, after that it receives the IP and all traffic occurs only through IP via TCP that requires seamless connections (resending information lost with ICMP control of the packets ).

Understanding the reason for opening the door

When a packet of unknown origin arrives at the router, it looks in its NAT table, who asked for it, and not finding it, the packet is discarded, replying with "Destination not reachable" or depending on the configuration, without answering anything, in the case of firewall.

The router has no crystal ball, it has several computers on its subnet, it would not forward the packet to everyone.

When we open a port, we tell the NAT of the router: I want everything that comes from the internet through port X, by unknown address, to reach me for that application on the computer 192.168.0.3 (or any other IP on the subnet).

That is, when you open a door, anyone on the Internet can communicate directly with your computer, going through the router without any problem.

In the case of P2P programs, this feature is useful as many customers release a higher download speed, only if the person has the door open (upload high).

There are also people who prefer to have servers in their own general purpose home, such as databases, web servers (websites), and more.

Of course, currently, operators also block certain ports for residential customers, that is, there is no point in opening the Web Server port on the router, and your ISP blocks it before this.

The ports usually blocked by ISPs are:

21 (FTP)

22 (SSH remote access)

23 (Telnet)

25 (SMTP, or e-mail trigger service)

53 / UDP (DNS)

80 (Web Server)

137 / UDP (NETBIOS Name resolution of computers on the network)

138 / UDP (NETBIOS Datagram)

139 (NETBIOS Session)

445 (Microsoft-DS or say, user authentication)

The telephone company also blocks ports: 1352, 1503, 1720 and 5631, some of which are NetMeeting meeting calls and remote access.

Live TIM in São Paulo also blocks port 3128, known for use as a proxy server.

To open all ports on your router to your computer, you can put it in DMZ (be careful, your computer can be very vulnerable, make sure you use a computer that does not have important data, or a temporary virtual machine).

Then we can inform the router that everything unknown will go to a certain address.

This is useful precisely to know which ports are blocked by your ISP, in case you plan to install a server using a specific internet connection.

Also remember that IP addresses on the Internet often change from time to time or when you restart the router.

Releasing a port for use

You need to see in your router's manual how to do this, usually in the "Virtual Server" sessions on the router you will find information on directing traffic from a specific port to an IP address.

When using DMZ feature, you just enter the IP address, so you don't have to worry about the port as it will be directing all unknown content to this address.

You also need to release the port on the Windows firewall for the traffic to work.

When opening the door, you can test using the following websites:

http://www.t1shopper.com/tools/port-scan/

http://www.whatsmyip.org/port-scanner/

http://mxtoolbox.com/PortScan.aspx

https://pentest-tools.com/discovery-probing/tcp-port-scanner-online-nmap

http://www.yougetsignal.com/tools/open-ports/

http://www.seomastering.com/port-scanner.php

http://incloak.com/ports/

These are some of the sites, there are several using the term "Online Port-Scanner" on Google.