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.
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.
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
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.
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 ftp.novell.com ) 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.
Bring up a DOS box and use doskey to save typing when you have to repeat commands
Use ping ftp.novell.com to send a few packets out onto the Internet
Use arp -a to inspect your ARP cache. Because you have just contacted Novell you should have an entry for the router connecting you to the outside world. This is, in fact, the answer to the question I have asked but what your need to do is to learn how to extract the same information from NetXRay.
Set up a capture filter that only captures IP/ICMP packets.
To do this use Capture/Filter Settings/Advanced Filter and then hit the "+" beside IP. You will see all the different IP protocols listed. Select ICMP.Type ping ftp.novell.com in your DOS window but do not hit enter
Start a capture and immediately hit <enter> in the DOS window
View the captured packets
Select a packet that was sent by your computer
In the packet decode look at the first header and note down the MAC address of the router that this was being sent to.
Harder – figure out the IP address of the NIC that you have identified above.
In order to see the IP address of your router in NetXRay you will need to capture an ARP request that was sent by your PC to learn the MAC address of the router.
If arp -a shows that you already have an entry for your router ( the answer you are looking for ! ) then you should delete this entry using:
arp -d <the IP address>
Set up a NetXRay filter that only captures IP ARP. ( this is a top-level protocol in the Advanced Filter window )
Type ping ftp.novell.com in your DOS window but do not hit enter
Start a capture and immediately hit <enter> in the DOS window
Look at the ARP transaction ( request fololowed by response ) and see if you can locate the data you are looking for.
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.
Display ( or redisplay ) the NetXRay Host Table window ( Tools/Host Table) and select the 'MAC' tab. Wait until you see the MAC address of your router in the list ( you can use ping ftp.novell.com to force the issue )
When the address is displayed click on the line in the host table and then use the filter button to automatically create a filter using that address. Alternatively bring up the Address Filter screen from the capture window and then locate the MAC address of your router in the Host Table within the capture windows.
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:
Use Packet/Display Options/Protocol Display to turn off the opening of all protocols ( "Close All" ). This will cause your capture buffer display to open in a condensed summary form by default.
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.