I have been trying to find a source that will show exactly how a packet behaves from source to destination as far as the Mac and IP addresses as it travels through its myriad of routers.
I have my CCNA and went through Ciscos academy, but it really didn't show (with pictures) how the packet changed at each step.
I know that an arp is done at first and if the host is not on the network, the router puts its Mac address in the source Mac address. And this happens, I believe, each time it hits another router. I have always been confused as to how it finds its way back if the Macs and IPs are changing. And if the Mac changes, why is it needed in the first place.
I'd love to find a place that shows the packets journey through about 5 different routers and see how it works (similar to the Nat diagrams here - which are great - never knew there were 3 types of Nats until I got here).
Changing Mac and IPs in packets
15 years 8 months ago #236
Your question is a very good one and I'm considering creating a page that will show exactly what happens to a packet as it travels from a LAN to a WAN. It will certainly help clear all these questions.
Even though the CCNA is an excellent certification, it does leave a lot of questions and gaps.
Coming to your question, MAC addresses exist on the 2nd Layer of the OSI Model. As you would already know, its used to uniquely identify a host in a LAN. When the frame arrives to a router in order to be routeded through a WAN link, the 2nd layer information is stripped, leaving the packet with the 3rd layer and above. This packet is then encapsulated into the data portion of the WAN's frame.
The important step here that you need to understand is that each WAN protocol has a different frame structure, which is why the original packet from your lan must be stripped from all unwanted information (such as the type the datalink layer embedded).
When the packet arrives to the destination LAN, the router there will strip all unwanted WAN information and encapsulate the packet into an Ethernet frame, using the MAC address of the destined local host.
I hope that helps you, rather than confuse you more!
I look forward to your page on what happens to a packet as it travels from a LAN to a WAN. I would love to see it go through multiple routers, including those on the LAN besides the WAN (if they are handled differently). I have about 10 books on the subject and none have this.
In the CCNA curriculum, it mentions that an ARP is done on an IP address, and if no one responds, the gateway router will take it and replace the the MAC address (I can't remember if it is the source or destination) with its own, and passes it on to the next network (LAN or WAN). I didn't realize that the whole Frame was actually stripped and a new Frame was added (although that does make some sense). I would figure that there would be another level of encapsulation (but then I suppose that would violate that OSI model).
That is why I am looking for different views on the subject, as it helps to clarify the question.
Changing Mac and IPs in packets
15 years 8 months ago #238
We will do our best to get the section covered asap, but you should be aware that there are heaps of other topics on the waiting list!
Be patient and bare with us, and lastly be sure that the result will be better than what you would expect. This is the reason new topics take some time to show up, because we try to go into a much detail as possible.
I too have found books that cover various networking concepts, not go into great depth in certain topics, and I'm not sure why. This is why this site was created, to fill in all them gaps and become the No.1 networking resource in the world [img]images/smiles/icon_smile.gif