INT21CN Computer Networks

Practical #12

1. Discover the IP address of the computer you're using to read this Prac sheet. How you do this depends on what operating system you're running.
   • On MS Windows, the only (built-in) way that I know of is to start up a "Command Prompt" (the thing that used to be called a DOS Window) and type the command ipconfig, or ipconfig /all if you want to know everything. There are lots of freebie GUI apps that Windows users can download that will tell them the same information -- most net-based gamers do this sort of thing all the time!

   • on a MacOS (9.2 and earlier) system you can simply look at the "TCP/IP" Control Panel. On MacOS X, the information is in the "Network" option of the System Preferences utility. The ifconfig works as well in a terminal window, see next bullet point.

   • The generic Unix command for this is ifconfig -a. This lists IP addresses, and lots more, for all network interfaces on the machine. NB: this command doesn't work on SGI systems. Instead, you have to specify which interface you want to know about. Typically we use ifconfig ef0 to find out about interface ef0.

   What is the (apparent) class of the IP address that you discover? What is the netmask in use? Are there any other IP addresses mentioned that are interesting? And anyway, why did I use the word "apparent" above?

2. Optional Exercise: write ^[1] a computer program that can translate between 32-bit numbers and dotted decimal form. Extend your program to determine whether the address is class A, B or C, and print the network and host portions. Further extend your program to recognise and identify special IP addresses such as the limited broadcast address, a directed broadcast address, etc.