CO285: Introduction to Networks

Assignment 7: Ports and Services

Due: October 15, 1998

1. Ports and services

   You can get a feel for some of the services offered on a TCP/IP network by using the telnet program to connect to various ports. Telnet itself just provides an keyboard/screen interface, whereby the characters you type are sent to some remote host and its responses are sent to your local machine. By default, the telnet program connects to port 23 on a host, which connects to a telnet server. This server receives characters, executes them on the local operating system, collects all the output and sends it back to the client.

   However, the telnet program on the client doesn't have to connect to the telnet server - it can connect to any of the defined ports on a host!

   For example, you can try to connect to a host on port 13. You should get the time sent back to your screen (some folks would use a wrist watch, but this is high tech). On vax1, this is done with the command:

   $ telnet vax1/port=13

   You can also set up the telnet on your PC to do this.

2. Try it on your own

   Try connecting to various ports on vax1.rockhurst.edu, teach.millard.org, and some other machines off campus, like www.theonion.com, and acm.org. Not all computers have all services enabled, and furthermore, many organizations may not even allow messages to these ports to get past a firewall, but it doesn't hurt to look.

   See if you can figure out what services are on each of the following ports: 7, 11, 13, 19, 21, 79, and 80. For some you will have to type something to send to the server, for others you won't. For one of these, try entering GET /.

Summary of Tasks

Turn in:

1. Write down what kind of server is on each port (it's the same service on all computers), and which machines (of at least 4) have that service available. (Hint: Everything should work for vax1.)