19.3 Java4630: the inetaddress class

Table of contents

• Preface
  • Viewing tip
    • Figures
    • Listings
• General background information
• Discussion and sample code
• Run the program
• What's next?
• Miscellaneous
• Complete program listing

Preface

This module is one in a collection of modules designed for teaching INEW 2338 Advanced Java (Web) at Austin Community College in Austin, TX. More specifically, it is one in a sub-collection of modules designed forteaching network programming in that course. The purpose of this module is to introduce the student to the InetAddress class that can be used to deal with IP addresses and domain names .

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand Listings while you are reading about them.

Figures

• Figure 1 . Program output.
• Figure 2 . The list of Google IP addresses.
• Figure 3 . The IP Address for my localhost.
• Figure 4 . Name and IP address of my localhost.
• Figure 5 . Canonical host name for Google.
• Figure 6 . Google host name using reverse lookup.

Listings

• Listing 1 . Beginning of the program.
• Listing 2 . Display the Google InetAddress objects.
• Listing 3 . Get InetAddress object for localhost.
• Listing 4 . Extract name and IP address of my localhost.
• Listing 5 . Get and display canonical host name for Google.
• Listing 6 . Do a reverse lookup on Google.
• Listing 7 . End of the program.
• Listing 8 . Complete program listing.

General background information

Every computer attached to an IP network has a unique 32-bit or 128-bit IP address.

For human consumption, we usually convert each of the bytes in the IP address to an unsigned decimal value and display them connected by periods to make them easier toremember. As of this writing, the IP address of www.austincc.edu (the college where I teach) is 206.77.150.222.

The domain name

What do we mean when we speak of www.austincc.edu ?

Each IP address can have a corresponding name known as a domain name . The domain name for the IP address 206.77.150.222 is www.austincc.edu .

For example, I can enter either the IP address or the domain name into the address field of mybrowser and use it to connect to the ACC web site.

The Domain Name System (DNS)

The Domain Name System (DNS) was developed to translate between IP addresses and domain names. Whenever you log your browser onto the internet andattempt to connect to a server using its domain name , the browser first communicates with a DNS server to learn the corresponding numeric IPaddress. The numeric IP address is encapsulated into the data packets and used by the internet protocol to route those packets from the source to thedestination.

The InetAddress class

We will learn how to use methods of the Java InetAddress class to find the IP address corresponding to a domain name in this module.

We will learn how to find the canonical host name for a given domain name.

We will learn how to obtain information about the localhost .

We will learn how to do a reverse lookup to find the canonical host name associated with an IP address.