Guide to routing for use at home, part 1 - understanding IP

When I am visiting a Client site, and I quickly open up a window on my macbook pro to remote control my home pc to retrieve some live information, people are often impressed... Or, when I am on the phone discussing a site change with someone, and they are able to see the changes 'live' on their PC, people are often equally dumbfounded. It's often the assumption that this sort of trickery is expensive and/or complicated to achieve. This couldn't be further from the truth. All it takes is a BASIC understanding of TCP/IP and a router with certain functionality and a little time!

As with most technical things, the first step is to understand them...

How do domains work and what is my IP address?

A good way to understand this is to understand web sites. When you type 'www.google.co.uk' into your browser window,your computer asks 'what is the address of google.co.uk for web browsing?'. The connected computer (usually your host, i.e. BT) (or other DNS server) returns '216.239.59.104'. This is the 'IP address' that the domain name 'google.co.uk' points to for the purposes of web browsing. You can try this, enter 'http://216.239.59.104/' into your browser... what comes up? google.co.uk. Your computer then retrieves information from the address 'http://216.239.59.104/' on port 80, which is the usual one for web browsing. So, google is actually a string of numbers, and the google.co.uk bit is just like a signpost that always points to whatever numbers happen to be giving the google service.

So, a domain name is simply a replacement or an alias for a string of numbers called an IP address!

Now you have to understand that everyone connected to the Internet has an IP address. That is not every computer however, but every point of contact. A router usually only has one IP address. This is often known as the WAN address. You can find your own WAN address by going to http://www.whatismyip.com. That is the address of your router or computer from the outside world. If you have 2 or more computers connected to a router, you will find that they have the same WAN address!

So, it follows that anyone typing in http://xxx.xxx.xxx.xxx with your WAN address will get through to your router... That's what firewalls are for!

By default, most routers refuse all incoming connections, i.e. anyone visiting your WAN address will not get any sort of response. But, you may want to selectively access something inside your network. That is called configuring your 'firewall', more on that later...

Static / Dynamic IP.

Some Internet providers offer static IP addresses. This simply means that every time you connect your router, you will always have the same IP address or WAN address. Most however are dynamic. This means that periodically (usually when you restart your router) the WAN address will change to a different one as assigned by your Internet provider.

What is a Port?

Remember that a web site is usually on port 80? Well, a port is like a virtual address within an IP address. There are loads of them, 65,536 of them with standard TCP/IP, and even more in the future!  Most 'services' have a specific port that over time has become the norm.  Lots of things can be services, but we will focus on three; port 80, 21 and 3389.  These ports are the standard for:

80 - web browsing

21- FTP

3389 - Windows terminal services

Read part 2 to find out how to use these ports to do useful things....