Client Server Communication

Routers

There are thus tens of thousands of interconnect points where dozens or even thousands of wires converge. The devices at these points of convergence are called routers. Routers know where other routers are and they route message packets onto the right wires. You likely have a small router in your office, it combines all the signals on all the network cables connected to it into one wire going out to the internet.

Ports

Servers listen for requests on a software mechanism called a port. For example, 21 is standardized as FTP, MySQL is 3306, web servers talk by default on port 80. These and other port designations are agreed upon by vendors, there are hundreds, even thousands. But we only need to worry about a few. Each message packet traveled on internet cabling has a label on the front of it (a header) specifying what destination computer it is headed for and what port it wants to go in through when it gets to there.

IP Addresses

For purposes of this discussion we can suppose that every computer on the internet has a unique Internet Protocol address (IP address). An address looks like this: 192.168.1.250.

Even if your computer is not connected to the internet, it still has a default IP Address assigned by the operating system when it starts up (assuming TCP/IP networking is installed).

If you have a web server running your client computer you can visit pages on it by typing the IP Address of your own computer into the address field of your browser. In addition, your computer assigns a textual address when it starts so you can also type "http://localhost". What if the web server is running on another computer in your office? You simply type the IP Address of that computer into the address field of your browser. You can thus have your own little two-computer internet by running a web server on one computer and a client on the other.

Protocols

The headers of message packets flying around the internet at any given instant not only contain information on the destination address and port but also the protocol. If the message is part of a web page the protocol is http (hyper text transport protocol).

Domain Names

Domain Name Servers (DNS) on the internet map URLs (like Digitalfire.com) to IP addresses. For example, if you type http://digitalfire.com into the address field of your browser it will ask a name server for the matching IP Address and then go there.

URLs

A uniform resource locator is a non ambiguous way of representing the protocol, domain name and port through which you want to ask a server for something. For example, for web access you normally want to ask the server for a specific page (or file). Consider the url http://www.digitalfire.com/index.html:80.

http represents the protocol, digitalfire.com is the domain name (which gets translated to an IP address), index.html is the file being sought and 80 is the port. Since 80 is the standard, you do not need to include it. Since index.html is the default home page for a website it does not need to be specified either. Also, since http is the default protocol for web browsers, it too can be omitted. In addition, since digitalfire.com is registered as both digitalfire.com and www.digitalfire.com, you can omit the www. That leaves just digitalfire.com, that is all you have to enter into the address blank of the browser.

Only custom-written web components can go down any road you choose!

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