Confusion over Ethernet - TCP/IP Model - Link Layer or Internet layer

[siva04301978]

Hello experts

Im novice to Networking.This Question could sound silly but thought the experts here can clarify my doubt better

I see that all the documentation around TCP/IP Model state that Ethernet is part of Data link layer and routing protocols (routers) are part of Network layer. When i look at the way my computer is connected to LAN and compare with TCP/IP layers  I see that my computer is first connected to Ethernet and Ethernet in turn is connected to the router, it appears as if data moves from link layer to internet layer and sounds contradictory when compared with the TCP/IP stack where internet layer is placed on top of link layer

can someone please clarify my confusion or direct me to better documentation. 

Thanks in advance

[Cisco Freak]

Hi Siva,

Hope this pic will clear your doubt.

[siva04301978]

Thanks jon & CF for the quick replies. 

Jon, I did the same, visualized the Stack from sending and receiving perspective. for eg : when i request for a web resource using http the request (data) seem to be flowing from application layer (http) >> Transport >> Ethernet (Link Layer)>> router(Network).  similarly when I receive the data back it seem to me as if its flowing from the router (network layer)>> Ethernet(link layer)>> Transport >> http(application)

Sorry Guys not sure if im missing something or over thinking the TCP/IP concepts. just wanted to get the concept right before i move further.

Thanks

[Jon Marshall]

No need to apologise, this is what the forums are for.

Are you okay with it now (sorry I can't tell) or do you still have doubts ?

Jon

[siva04301978]

Jon still im left with a doubt.. I have just updated my earlier reply further more to keep it clear. I have put up a eg to express the way im thinking/looking at the stack. Highlighted my doubt in bold

eg : when i request for a web resource using HTTP the request (data) seem to be flowing from application layer (http) >> Transport >> Ethernet (Link Layer)>> router(Network).  similarly when I receive the data back it seem to me as if its flowing from the router (network layer)>> Ethernet(link layer)>> Transport >> http(application)

[Jon Marshall]

Okay I think I understand what you are saying.

Firstly I assume the web server is not running on the router ie. you are not accessing the router.

This I think is where the confusion is coming in.

A router does most of it's work at the physical/data link/network layer. Note that if you telnet to a router then it goes further up the stack but for this example we will assume your client is requesting a web page from a web server that is on the other side of the router ie. a different network.

So the router is directly connected to the client on one interface and the web server on another .

So your client sends a packet to the web server. It travels down the TCP/IP stack to the physical layer and is sent on the wire to the router.

Now the router here is simply forwarding traffic so it does not need to pass the packet all the way up it's TCP/IP stack. It receives the packet at the physical layer, strips the mac address from the packet and assuming it is the routers mac address then looks at the destination IP.

So the highest the packet goes on the router is to the network layer.

It then does a route lookup, adds a new mac address (the servers) and put's the packet back on the wire and sends it out ie. the packet has travelled back down from the network layer to the data link layer to the physical layer.

When the web server receives it the packet will go all the way up the TCP/IP stack to application layer. The response will then travel all the way down the web server's TCP/IP stack and be sent back to the router which will do what it did before and then send it to the client.

Does that help ?

If not or you have more queries then again feel free to ask.

Jon

[siva04301978]

Perfect! Thanks jon that explains.

in addition to what u explained i just found an article which helped me in correlation with Ur explanantion

http://www.windowsnetworking.com/articles-tutorials/trouble/Making-Sense-Windows-Routing-Tables.html

Thanks

[Jon Marshall]

You have to think about it from the perspective of sending and receiving.

So for the sending device the data moves from the top down ie. it goes from the application layer downwards through the layers to the physical layer.

But for a receiving device it happens in reverse ie. it goes from the physical layer up through the layers to the application layer.

The TCP/IP stack is just showing you the separate layers but the direction depends entirely on whether your PC is sending or receiving data.

Does this make sense ?

If it doesn't or you need more clarification please feel free to ask further.

Jon