CS1102 Lab 8

This week's work

Below, there are 4 activities. Complete each activity, and answer the questions. Write your answers onto a sheet of paper. Write your name on the paper. Hand it up at the end of the session. After you have completed the activities, resume work on your personal home page (lab 6).

Re-read the worksheet for lab 7, if you need to remind yourself of how to run nslookup and ping.

Let me remind you again of UCC's Acceptable Usage Policy.

Activity 1

(Windows or cosmos) As you know from lectures, there are 232 IP addresses (IPv4) but many of them are wasted and not in use. (Do you remember why?) Let's use nslookup to make a (rather unreliable) estimate of the proportion that are in use.

Invent 10 different IP addresses, at random (but make them fairly different). (Remember, each IP address consists of four decimal numbers, each between 0 and 255.) Use nslookup on each of your 10 IP addresses. If the address has a domain name, we'll assume the address is in use. (Reasonable enough!) If the address doesn't have a domain name, we'll assume the address is not in use. (Dodgy!).

  1. On this basis, what is your estimate of the proportion of the IP address space that is in use?
  2. Why was this unreliable?

Activity 2

(cosmos) Choose the hostname of some computer on the Internet outside of UCC. Use nslookup to find its IP address. Now ping the hostname, and then ping the IP address. (Some machines disallow ping requests, in which case you'll have to pick a different target computer.)

  1. Do you see any difference between pinging the address versus pinging the name? If so, what?
  2. Why would there be a difference?

Activity 3

The whois command can tell you to whom a domain name is registered.

  1. First use nslookup to compare the IP address for britannica.com and the IP address for britanica.com. Then use whois to find to whom these doman names are registered. What do you think is going on here?
  2. Do the same for w3c.org, w3c.net and w3c.com. What do you think is going on here? (Use your browser to visit these hosts, if it helps.)

Activity 4

(cosmos) You can use /usr/sbin/traceroute to obtain the route that a packet follows across the Internet, e.g.: 

/usr/sbin/traceroute www.w3c.org

Let's use /usr/sbin/traceroute to estimate the diameter of the Internet.

If we were being mathematically accurate, we would measure the diameter by finding the shortest distance (in terms of hops, not kilometres) between every pair of computers. The diameter would then be the longest of these paths. The longest shortest path, if you like!

But we can't do that.

Here's what we'll do. Use your browser and find 5 web sites that you think are hosted a long way from Cork - companies or universities on the other side of the world, that sort of thing. On each of these, run /usr/sbin/traceroute. In each case, count the number of hops. (If the output of traceroute includes *** entries, ignore them in your hop-count. On the other hand, if traceroute starts to display indiviudal asterisks at a rather slow rate, then it is 'getting stuck', in which case choose a different web site.)

  1. What's your largest hop-count?
  2. What URL was this for?