Addressing & Routing

Now that we know how to send post cards (UDP packets) and make telephone calls (TCP connections,) how do we specify who we want to talk to? And, once we specify who we want to talk to, how does the message get from where we are to where it's supposed to go?

Routing

The address information that a human writes on a post card tends to follow a certain form: Street number, Street, Borough, City, State and Country. Notice how the address is written in human form: From the nearest details to the farthest details. When sent to a different country, the post card address is processed in the reverse order: At each step in the delivery process, the size of the next geographical region is reduced.

As the post card is routed at the various steps before the final destination, the people who are routing it don't really know exactly where it's going - but each one can figure out how to get it to the next step in the route. Finally, at the last step, the mail man who is handed the post card actually knows where to find the house that the post card is going to.

TCP/IP networks also have an addressing scheme and a process for routing a message. Addressing of devices on a TCP/IP network is not much different from telephone numbers. The Routing procedure on the network is more complicated than the one used by the Post Office - but the basic ideas are not much different. Messages need to be routed from one network to another until they arrive at their destination. At any given step in the chain there may not be enough information available to know exactly where the message will arrive - but each router in the system does know the next step in the route that the message must take.

Fortunately, a typical server in a small company or home office is not connected to many networks. As a result, routing is often very easy to do for the majority of network administrators. For now it's enough to know that messages do need to be routed - we don't need to study routing in more detail just yet.

Addressing

On the Internet today there are two major addressing systems available: Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6.)

The one that most of us are using at the time of this writing is IPv4. It allows a message to be delivered to any one of about four billion devices. This may sound like a large number, but, because of the way that IP numbers have been assigned to various companies throughout the world, we don't have enough to go around. There is currently a shortage of IP numbers.

IPv6 solves this problem. IPv6 supports so many possible devices that there's no real chance that we will ever run out of IP numbers in the future. When will start using IPv6? Well, at the moment there is more and more software available that is able to operate in IPv6 environments. We should be able to go ahead and start using it right away. Eventually more people will do so and, eventually, the day will come when you will connect to your Internet Service Providers' network and find that you have been assigned an IPv6 address. On that day you will know that the IPv6 future has arrived, at least for you.

For learning purposes we can continue to use the shorter IPv4 numbers. Again, we don't really need to know about routing just yet - but we need to look at addressing.

Each computer (specifically each network device) needs to have an address so that it can be found. The device address is called an Internet Protocol number, or IP Number. To make it easier to remember it's broken up into sections - sort of like a telephone number - but basically it's a big number.

IP Addresses are also logically organized into ranges. Some of the IP ranges are reserved for internal use within private networks. Other ranges are assigned to various companies, on a first-come first-served basis, by an international organization that was charged with this responsibility. Most of us get our public IP numbers from our local Internet Service Provider.

Port Numbers

Computers tend to run lots of software at the same time. When we address a message to another computer, we need to specify which program we want the message to go to. This part of the address is called the Port Number. It is used to route the message to the software that will handle it.

Destination Port Numbers for common protocols are assigned by convention. For example: if we want to get a web page from a web server we connect to Destination Port 80. Port 80 is the well known port number for the web protocol. The http:// part of the address that you type into the browser eventually translates into port 80 - you don't often need to type the actual port number. Another well known port number is 25 for sending email. Again, you don't normally need to know the number - by convention the network software defaults to port 25 when email is being sent.

There are 65,535 port numbers available. The standard port numbers are listed in a file called services. On Unix systems this file is found in the /etc/ directory; in Microsoft Windows it's found in the windows installation directory (often C:\Windows\.)

Source Port Numbers are used for responses sent back from the server. When a web browser connects to a web server it must specify destination port 80. Remember that, by convention, port 80 is the well known port number that the web server software is listening to. However, the web browser doesn't care what port number it receives its replies on. For this reason, Source Port Numbers are automatically assigned from available port numbers when a connection is made.