In this lesson you will explore the benefits (and potential security concerns) associated with routing traffic across the Internet. Building on the introduction to IP addresses in the previous lesson, you will simulate the Internet in groups that allow messages to be sent only to an intended recipient, as indicated by an IP address. This simulation will also allow you to examine the traffic that goes through all of the (simulated) routers on the network. You will see that messages go through many different routers, may not always take the same path to reach the final destinaton, and that the routers (and their owners) can see all of this traffic!



Students will be able to:


The main purpose here is to give students a taste for both the scale and implication of what routers do by examining the network traffic that is generated by students in the class. Seeing traffic like this in real life is very hard if not impossible since only engineers of large networks have access to router data on a large scale, and it's also more complicated to understand. Our "network" is simulated and provides a simple view, the basics are the same, and quick investigations reveal interesting things.

CS Background: A router is a computer designed to receive and redirect packets of information. based upon the addressing information (e.g. an IP address) contained in the packet. Routers will either deliver a packet to its final desination or forward it to one of several other routers it is connected to. By monitoring current network conditions, a router can determine which of these will allow the packet to reach its destination fastest. There will often be redundant paths between two locations on the Internet, and so if one path is experiencing traffic or otherwise out of service, additional paths will be available. This redundancy makes the Internet more reliable and also helps the Internet to scale, accommodating new users (and routers!) as they are connected to the system.

Getting Started

Imagine you were going to send a letter to a friend living in another state. List the steps you imagine your letter would have to take through the different parts of the postal system. Don't worry if you're not sure about your answers, just make an educated guess.

If possible, discuss your answer with antoher person. Below is an example answer. How is your answer similar to the example? How is it different?

Pro Tip

The exact details matter less than identifying multiple points where the letter is sorted and rerouted based on its intended destination. No need to go into great detail here, but the hope is to help students develop a mental model for how a system for routing messages looks and what role a router serves.

When we send messages through a network we don’t actually want everyone on the network to receive them. If we include information about who the message is intended for then we can allow portions of the network to focus on sorting and routing messages, so that they can continue on their way to their intended target. In the mail system, mail facilities, post offices, or a mail carrier fills this role.

In a network of computers, certain computers called "routers" do the same thing, directing messages towards the target computer based on the IP addresses included in the message.


Today we will be simulating a network. Students should be in groups for this activity, with each group centered around a shared "router". Assign each student in the group a different 8-bit "IP address", such as "1.7 (0001 0111)" or "5.14 (0101 1110)" and give each student several index cards or another way of communicating separate messages. (Every student in the class should have a separate and unique IP address for this simulation). Alternatively, students could send emails or text messages to the teacher to be "routed" (forwarded) to its final destination. Each message should include a "To" and "From" address field which will allow for messages to be sent to a single intended recipient. This simulation will ask the teacher to route messages across a network, with messages possibly being routed across multiple routers (groups) in unpredictable sequences before finally being delivered to the intended "IP address". This is done to simulate the way traffic travelling across the Internet is constantly rebalanced in response to over-usage or under-usage of some channels. A message will usually make it to its destination, but we can't know for sure how it will get there.

Teacher Tip:

Alternatively, students could send emails or text messages to the teacher to be "routed" (forwarded) to its final destination.

Send a quick test message: Have students send a simple "hello" to a classmate who is connected to the same router.

Teacher Tip:

As you are routing messages to their final destination, take different paths to get there, drop some of the messages, and read some messages aloud to enforce the notion that messages may take different paths to the same location, some messages may be dropped, and routers can theoretically read all traffic moving across them.

Have a Conversation: Ask students to conduct a short conversation (e.g. a simple greeting or a question and answer) with two classmates on their "router". They should verbally confirm that both sides are receiving their messages. Provide students a few minutes to practice. Help students construct their messages. If their "To" address is not constructed properly the message will be dropped (not delivered).

Transitional Remarks:

Find a Classmate on a Different Router: Ask students to find two classmates on a different router and ask for their IP address (they will need to actually talk in order to do this). Again students should conduct a short conversation with their two partners, confirming verbally that the messages are being received.

Explaining Redundancy: The simulations students used in today's lesson model many features of the actual Internet, most notably its redundant nature (there are many paths between locations). Assist students in identifying these features with the following comments:

Wrap Up

Ask students to answer these reflection questions:

Discuss: Allow students to share their answers, either in groups or with the class. The discussion should touch on:

Teaching Tip:

Here's a cheat sheet for the features of the Internet students saw today and how today's activity is simulating an actual feature of the Internet:

To / From Address: Like an IP address, included on every message sent over the Internet.
Dropped Messages: Poorly formed messages cannot be delivered and so are dropped, just like a letter with a bad address on it. Tomorrow you'll discuss more technical reasons messages are dropped.
Multiple Hops: A message travelling across the Internet will visit many routers as each tries to forward it along the most efficient path to its destination.
Different Paths: Routers respond to traffic on the Internet in real time. The best path at one moment might be backed up a few seconds later. Routers choose the current best path to get the message through.

Note: In this simulation the "hops" are chosen randomly. Actual routers use algorithms to determine the best path to send a message along.

Standards Alignment