Lab 4: Remote Delivery

There are three parts to this week's lab – an initial exercise using NetSim to build routing tables, a brief interlude with NetXRay and lastly a protocol design exercise.

NetSim exercise

Here you will be using NetSim to build up an extended network – a simulated Internet. In this lab NetSim will be running "Lesson 6" and will allow you to add two cards and therefore be connected to two networks. Using this ability you will be able to build various network configurations and then create routing tables that will enable communication to take place through the networks you have built.

Simple network

The network you will build will look like this – each computer will have two network cards – one connected to each of the networks ( "left" and "right" ). To do this foloow these instructions:
Name your computer & hit <tab>
Ask for 2 network cards and hit <enter>
Connect card 1 to "left" and card 2 to "right"
Write the name of your computer on the board

The consequence of this arrangement will be that each computer knows how to contact both networks – because the networks are directly connected to each computer. You should then be able to ping or chat with anyone in the room. To do this follow these instructions:
Pick another NetSim computer to ping
Put the name of one of their NICs in the Destination Node box ( eg fred1 )
Put the name of the network that this NIC is connected to in the Destination Network box ( eg left )
Use Action/Computer/Ping to perform the ping

After this exercise restart NetSim

Complex network

Depending on how many NetSim computers are running build a network like the one shown below. This network has a backbone, to which a number of computers are connected and then a number of branch networks. In this scenario there will be numerous "remote" networks ( one's that you are not directly connected to ) and you will need to work as a group to make sure that everyone has appropriate routing tables built up.

To modify your routing table use the Edit Tables/Edit Routing Table menu item. When the routing table is displayed you can use the Insert button, the Delete button or the Edit button if you want to change an entry.

The object of the exercise is for computers at the extreme ends of this network to be able to communicate with each other.
Folow the tutor's instructions in setting up this network.

Give all computers names. Each time you connect a computer to a network that did not exist before you must give this network a name - call the networks "a", "b", " "c" etc for simplicity. Create as many branches as needed to involve everyone – you can be creative if you wish and build different shapes of networks - but do not create "loops" - these raise more complex issues.

As you add your computer to the internetwork show on the whiteboard where your computer fits in. Clearly write the name of your computer, the name of any new network you have created and the numbers of your interfaces ( 1 & 2 ). This is the information that others will need in order to build their routing tables.

NetXRay exercise

This exercise looks at the real network and asks you to figure out some information about your connection to the Internet using the packets that you see and decode. Your computer will be on a separate network ( a "sub net" ) with the some other device ( a server or a router ) connecting you to the Internet. If you ping a remote location ( eg ) the ping packets will have to travel out via this router..

Use NetXRay to figure out the MAC address of the NIC in the router that connects to this lab.
Harder – figure out the IP address of the NIC that you have identified above.
Now try creating a filter to capture only the IP traffic between your station and your router. You will need a filter with two components:

An Address Filter – this filter will only accept packets to and from your router. In this way you filter out any other local traffic between you and your peers. If you are working at home and dialled in there won't be any but you can still create the filter.
You can enter the MAC address of your router manually or else use the Host Table.
An Advanced Filter – this may be needed to restrict the capture to IP traffic. If your network is using any other protocols you may waste your "5 packet limit" on these other packets.

Before capturing you may like to modify the way that NetXRay displays packets in the following way:
Perform a capture whilst you browse a remote Web site and then examine the 5 packets that you capture. Review the source & destination MAC address for each packet along with the source and destination IP address. Get really clear about what is happening.

Protocol design exercise

In the NetSim exercise you will have needed a lot of person to person communication to get the routing tables correct.

Work in a group to design a protocol that would enable the computers to exchange routing table information automatically. You can use NetSim "chats" to try out some of your ideas.

You might choose to use either or both of the network tools to help you here:

Set up a filter that captures IP/UDP/RIP and wait a while to capture some RIP packets. Then you can see how it's actually done.

Use NetSim "chat" to relay your routing table to another NetSim node.