Mac – Theory regarding MAC and IP address in network packet and frame content

ip addressmac addressnetworking

Okay, network theory and protocol questions for you guys:

Assuming a router is using NAT, is a device's MAC address every actually exposed to the Internet? Or, does the router transmogrify (wow, spell-check didn't have a problem with that word, I always thought it was just a Calvin and Hobbes term) the MAC like it does with the source device's IP address?

I seem to remember from classes that once the packet hits the network router the router substitutes its IP and MAC info in there, slaps a frame on it and then kicks it out onto the web in the general direction of the closest IP match. However, a thread that I read on another site indicated that the MAC address changed with every hop!

While that may be nice for privacy issues (no real way to tell what the original device/routers MAC was from an arbitrary point along the hops), it doesn't really seem to make sense that both the IP and MAC would need to be stripped each time.

Best Answer

A mac address is only relevant to the local broadcast domain (vlan, subnet). The source MAC address on each packet is the MAC address of the device that emitted the packet onto the local network.

It is a layer 2 address. So it isn't so much the router "substitutes" its own MAC address, the only possible (for the sake of this discussion) mac address that can be on a packet emitted from the router is the mac address of the interface on the router that the packet originated when put out onto the internet.

Note that the router mac address is only present in the packet until it reaches the next hop - the next router. At that point the packet is routed, exits the ISP router, and has the MAC address of the ISP router interface.

None of this is about security or privacy, it is the difference between how layer 2 and layer 3 networking works.

Related Solutions

Mac – Change MAC address of a wireless network adapter

If your hardware doesn't support changing it's MAC address then the easiest solution would be to purchase a new USB 802.11x card. Since the MAC address depends on the NIC a new usb card would do the trick.

Edit:
You can change your MAC address by doing this.

From the command line, log in as root and type;

ifdown eth0
ifconfig eth0 hw ether 00:80:FF:FF:98:F5
ifup eth0

This change will only persist until you reboot, so you probably want to put this in your boot script /etc/init.d/

Credit : http://www.linuxforums.org/forum/suse-linux/24956-clone-mac-address.html

Networking – How does a frame retrieve the recipient’s MAC address

The sender transmits an ARP request (or in IPv6, a Neighbor Solicitation). Basically that is a frame which yells 'which MAC has this IP?'

The owner of that IP will then reply with an almost identical packet, stating 'I have this IP'. That answer also contains the MAC.

This only works on the local network. For all IP traffic not on the local network the packet gets sent to the default gateway. (And obviously, that default gateway is a computer on the local network).

Edit1: Link to the relevant Wikipedia article on ARP

Edit2: The ARP request 'who has this' is obviously a broadcast.

Best Answer

Related Solutions

Mac – Change MAC address of a wireless network adapter

Networking – How does a frame retrieve the recipient’s MAC address

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