19.1 Java4620: general information  (Page 4/8)

For human consumption, we usually convert the value of each of the bytes to an unsigned decimal value and display them connected by periods to make themeasier to remember. For example, as near as I can tell, as of this writing, the IP address of www.austincc.edu (the college where I teach) is 206.77.150.222 .

Domain Names

What do I mean by www.austincc.edu ?

Even though we can do some tricks to make the numeric IP addresses easier to remember, humans don't do a very good job of remembering long stringsof numbers. Humans remember words and names better. Therefore, most IP addresses have a corresponding name known as a domain name . The domain name for the IP address 206.77.150.222 is www.austincc.edu .

The Domain Name System (DNS) was developed to translate between IP addresses and domain names. Whenever you log your browser onto the internet and attempt to connect to a server using its domain name, the browser firstcommunicates with a DNS server to learn the corresponding numeric IP address. The numeric IP address (and not the domain name) is encapsulated into the data packets and used by the internet protocol to routethose packets from the source to the destination.

(You should also be able to enter 206.77.150.222 into the address field of your browser and access the college where I teach.)

We will learn how to use the Java InetAddress class to find the domain name corresponding to an IP address, and to find the IP address corresponding to a domain name.

What is Your IP Address?

Do you have an IP address and a domain name ?

If (like me) you use a commercial Internet Service Provider (ISP) for your Internet service at home,you probably don't have a fixed IP address or a fixed domain name . Rather, the ISP has a block of IP addresses reserved. When you subscribe to the ISP, the ISP temporarily assigns an IP address to you. That IP address may or may not change over time, but probably will change unless youpay the extra fee for a fixed IP address.

On the other hand, I currently pay for server space from a company in North Carolina under thedomain name www.dickbaldwin.com . My domain has an IP address of 98.129.229.162 . For as long as I continue to pay the bill, that domain name and that IP address will be assigned to me foruse as my personal website.

I suppose that the company could change the IP address at some point provided that they make certain that all of the domain name servers get updatedto reflect the new IP address that corresponds to the domain name.

Ports

Each server computer that you may connect to will be logically organized into ports . These are not physical ports in the sense of the VGA or HDMI port onthe back of your computer. Rather, they are simply logical sub-addresses which you provide to the operating system on the server so that the operating systemcan cause the appropriate server software to "answer the call." We will write a simple server software package that will service several different ports onindependent threads in a future module.

One of the Java books on my bookshelf refers to the IP address as being analogous to the telephone number of a company and the port to be analogous to the employee's telephone extension within that company. (At least that is how telephone systems in companies were organized when I was working in industry.)