MAC Addresses in routed datagrams

[robertfl00k]

Hi,

I am due to teach some pupils the basics of MAC Addresses/IP Addresses and routing soon and I have a (hopefully) quick question which I am hoping someone here can help me with.

When a frame is placed out onto a public network is has a destination MAC Address. Is this address the MAC address of the next hop or the address of the final destination. If it is the next hop then am I right in assuming that the once the frame reaches the next router, the frame is unpacked to obtain the IP address from the layer 3 headers, and a new layer 2 frame is assembled with the MAC address of the next router to which the frame is to be sent. 

[Mukesh Kumar]

Hello robertfl00k

When a frame is placed out onto a public network it will be having destination Mac Address of the next hop. When that frame will be received by the router, then it will remove the layer 2 information and check its routing table for the destination ip address. If destination will be available there in the routing table then it will attach new layer 2 information and forward that data to next hop.

Regards,
Mukesh Kumar
Network Engineer
Spooster IT Services

[Richard Burts]

An Ethernet MAC address identifies devices on the local network. If the IP destination is on the local network then the MAC address will be the address of the destination host. If the IP destination is remote then the MAC address will be the address of the next hop device which will forward the packet off of the local network and will build a new layer 2 header in the process. So the understanding by the original poster is correct.

HTH

Rick

[robertfl00k]

Thank you for the help/confirmation.

One other question. When a router obtains a packet and decides where to send it, it has to build a new layer 2 header with the new/next MAC address in it. As the router is adding/removing layer 2 information does this mean a router works on layer 2 as well as layer 3? 

I ask because some of the online explanations seem to infer that routers only work at layer 3 of the OSI model.

[Richard Burts]

It might seem that routers only work at layer 3 because so much of the explanation of what they do focuses on the layer 3 work that they do. But no router can work at layer 3 without also working at layer 2 (and at layer 1). A basic premise of our layered model (on which TCP/IP is based) is that for any layer to function that all of the lower layers must be functional and working. So as a router is working at layer 3 it must also be working at layer 2.

HTH

Rick