Remote Connections

I wrote this explanation for a client. The client had no IT staff. And they relied on the local cable company for their Internet connection. The six hour drive prohibited an on site visit.

Key terms...

IP ADDRESS

The public, numerical name of a computer.

PROTOCOL

The language used when two computers communicate.

PORT

A number identifying the protocol to the computer.

TELNET

A protocol used for real-time interaction with another computer.

ROUTER

Hardware device that connects multiple computers to the Internet.

You already know that the computers use an IP address when connecting. When I connect with your application server, I supply the server's IP address. The Internet recognizes two "types" of IP addresses: public and internal.

I connect through the public IP address. This is the IP address accessible to the entire world. It is assigned to you by your ISP. The public IP address accesses the router installed by the ISP.

Your office has many computers connected to the Internet - personal computers, the application server, etc. These connect to the router. They too have IP addresses. Their IP address begins with 192.168. 192.168 is a special prefix. It signals an internal IP address.

The router does not allow access to a machine with the 192.168 prefix. This prevents someone on the Internet from accessing the company's computers.

Along with the IP address, I also specify the telnet protocol. The protocol instructs the computers how to communicate. telnet provides commands for interacting with a remote computer as if I were sitting in front of it.

Sadly, computers do not understand neat names like telnet. They understand numbers. My computer translates telnet into a port number - 23. And then contacts your IP address saying I want to talk on port 23 (telnet).

Imagine a ship pulling into New York harbor. It docks at a port for unloading. The ship's captain speaks his lingo (protocol) with the harbor master for instructions. But one of those containers actually goes to Chicago. So the container ends up on a truck redirected to Chicago.

Comcast is doing something similar with your router. The router receives my request for port 23. The router redirects it to the internal IP address of the application server. And now I can remotely log into that machine to work.

In short, the ISP will configure the router to redirect port 23 (telnet) to the internal IP address of the application server.