03 September 2008

How Network Devices Communicate

Getting to know a little deep about how the network devices communicate in your LAN can be quite handy, especially if you're going to take the CCNA exam.

You can find many questions related to the how network devices communicate, what data unit they're using. You have to remember the data unit related to each layer in the OSI Layers.

From the upper layer generate the data, the transport layer turn this data into segments, the network layer divide them into packets, data link layer then divide again into frames, and lastly the physical layer will handle the bits.
You need to be familiar how each device treats the data unit.

Here is the typical question you'd get in the CCNA exam, look at the below picture:

As a network admin you have the above network diagram, you have 2 networks, and Host A sends a request to see email from the Mail Server, what are the destination and source of MAC address and IP address received by the Mail Server?

Now when facing this question we first need to know what will the Host A send to reach the Mail Server.
A Host in a network will only know about its own network, it doesn't know anything about the other network accept for the IP address. So Host A will send the following:

Source MAC : 00-14-BA-AA-AA-AA (the host MAC address)
Destination MAC : 00-14-BA-BB-BB-BB (the router's FA0/0 port MAC address)
Source IP :
Destination IP :

In term of MAC addresses, remember the host only knows the MAC address of its default gateway (router).
The host will send it to the switch, switch will examine the frame and send it to the port which the router attach to.
Switches always remember the MAC addresses attach to their ports, switches keep this information in its database.

The switch then send it to the router. Router receiving this packet will then examin the packet further. It will check the packet for corruption then strip off the MAC address.
Router will determine where it should forward the packet to, it will first add its own MAC address as the source address and the next hop (router or PC) MAC address as the destination address.
The source and destination IP address will not be altered.

So at last the Mail Server will receive the following information:

Source MAC : 00-14-BA-CC-CC-CC(the router's FA0/1 port MAC address)
Destination MAC : 00-14-BA-DD-DD-DD(the Mail Server's MAC address)
Source IP :
Destination IP :