Lab 2: Local delivery

In the lecture this week the local delivery of packets on a network was described. Local delivery usually depends on the hardware, or MAC ( Media Access Control ), addresses of the computers involved. Every single packet passing through the network in the lab is on its way between two computer systems connected to the lab cable and each packet will have the MAC address of its destination, and the MAC address of its source stored in its "header" ( the "front end" of the packet ).

The sending and receiving of packets by the PCs on a network is actually pretty automated. It is relatively difficult to manually manipulate the MAC addresses in real packets on the network which means that the first part of this lab is not based on the sending and receiving of real packets.

Instead we will investigate local delivery of packets using NetSim, an application that allows you to connect simulated computers to a simulated network. Once you have connected your "virtual computer" to a "network" you should be able to send messages to other students who are connected to the same network – provided you can correctly address the messages. Several more lab sessions will make use of NetSim in order to do things that would otherwise require physical re-organisation of the lab wiring.

NetSim has been developed by David Stratton. Please give feedback via email to – Subject: Netsim feedback.

NetSim exercise
( This exercise involves a simulation of a computer network with simulated computers, network cards and network cables. Any reference in this explantation to "computer", "network" or "network card" is a reference to the simulation of those objects )

The initial NetSim screen looks like this with the top portion of the screen representing your computer and the two lower sections representing two network cards that may be installed in your computer:

During this first lab session the window title bar should read "NetSim - Lesson 1" and in lesson 1 only one network card can be plugged into your computer.

To create your computer you must give it a name in the box provided - the name of your computer must be unique and it will help everyone if you stick to lower case only for your computer name. Tabbing out of the box will let you set the number of cards in your computer.

Notice that there is a trace window at the foot of the screen – look out for messages there that inform you about what is happening. Once you have traffic flowing you will see the contents of the packets displayed. Try to understand what you see.
You also need to say how many network cards are connected to your computer – in this lab the only valid response is one. Hit <enter> to confirm the number you have entered.

After the card has been added you can then connect this card to a "network" by using the "Connect" button but first you must enter a name for the network – left hand side of the room use "left" ( case sensitive! ) – right hand side of the room use "right"
When your card gets connected to the network it is given a "MAC Address" – the 12 hex-digit number will appear at the top of the screen. The MAC addresses of all the cards in a network are guaranteed to be unique.

By entering the MAC Address of (((AnotherNetSimUser))) network card in the "Destination" box you can then communicate with that student.
There are two options for communication:To make anything happen you need to use the 'Action' menu - actions can be either perfomed on the computer itself or on an individual network card. In the case of local delivery you need a card level action. Select Action/Card/Card1/Ping or Send.
Try communicating with a student on the other "network". What happens

Notice that the MAC Addresses have a first half ( six digits ) that has a number of fixed values ( there are ten possibilities in this simulation ) whereas the second half is random. The first six digits of a MAC Address are the Organisation Unique Identifier (OUI ). Use the IEEE web site to figure out what companies "own" the identifiers you are seeing.
If the connection fails try using the 'Refresh' or 'Reload' button in your browser.

Address resolution

By this stage you will probably have mistyped quite a few of these long MAC Addresses and be wondering how any network communication ever takes place in the real world. The answer is that MAC Addresses are generally not used by the human users of the network – humans are permitted to type in names for destinations and there are protocols that can translate these names into MAC Addresses.