Question about MAC address

When I am behind a router, and send some TCP/IP packet to the internet, is my MAC address transmitted as source or the MAC address of the router is transmitted.

I use ethernet to connect devices on local network. I use my own MAC addresses which are unique inside the network but are re-used in other networks. Is this any cause of concern.

Like there are private IP ranges of 192.168.x.x and 10.x.x.x, is there a range of private MAC addresses.

All outgoing Ethernet packets from Router use Router MAC addresses.

http://www.certsoft.com/mac.htm
Please note that the most significant byte of an Ethernet MAC address has two special bits in it. The LSB bit is set to one to indicate that this is a group address instead of an individual address. The next bit is zero if this is an universally administrated address (OUI purchased from IEEE). If this bit is one then the entire 48 bit address is locally administrated and no OUI is used.

http://wiki.ethereal.com/Ethernet
The second least significant bit of the first byte is the "Locally Administrated" bit. This bit is always set to 0 for all assigned OIDs. The purpose of this bit is that if you change your MAC address you should also set this bit to 1 in the new MAC address so that it is clear it is not a factory default MAC address. Many, but not all, cluster configurations that utilize MAC address failover will set this bit to 1 for the failover interface.

When you are behind a router you are using a non-routable private IP address. When you access the internet the router does Network Address Translation, this means that it switches your private IP address for the Public address of the router. It records the private IP address in a table. When the internet packet is sent out, the MAC address of the router is wrapped inside of the TCP packet that is transmitted.

The MAC address is not transmited anywhere because is at level 2 of the OSI model. This means that your MAC address is only known in your LAN. There is no problem if you want to use the same MAC in two differents LANs but the standard say that it is not posible to have the same MAC in two devices because MAC addresses are unique.

clik help-me if I helped you.

Regards.

(sorry for my English)

MAC addresses are unique for standard LAN adapters since they are made by companies who have MAC addresses alocated to them by some global authority. I am using ethernet to link devices in local networks. I dont have a library of MACs so I use my own random MAC addresses. Most of the time, these deveices are not even connected to the internet. But I was worried that if they are connected, would that be a problem.

techie said:

MAC addresses are unique for standard LAN adapters since they are made by companies who have MAC addresses alocated to them by some global authority. I am using ethernet to link devices in local networks. I dont have a library of MACs so I use my own random MAC addresses. Most of the time, these deveices are not even connected to the internet. But I was worried that if they are connected, would that be a problem.

Click to expand...

Only problem occur, if we have two deveces with identical MAC addreses in the same ethernet network segment (switched, bridged), and maybe to router different ports directly.

When you are connected to a LAN your MAC ID is visible to the local LAN members only.
This is because MAC ID belongs to the lower layers of communication (TCP/IP).
Usually all other computers connected to your router will be seeing routers MAC ID as the origin.

But depending on the type of internet connection you are using others in the internet can see either your IP Address or routers Address. If you are using some kind of NAT or PROXY they will be seeing router IP as source of communication.
Otherwise if you are communicating with your own dedicated IP Address then they will see your IP as the source. This is because IP exists in a layer spread across all connected users, But MAC layer is spread only among the LAN.

Anyway even if your MAC ID dupliation occures most of the new NICs can use user specified MAC ID. For example check RTL8139 based NIC. You can change your MAC ID from NIC properties in Windows (In LINUX too...).

techie said:

When I am behind a router, and send some TCP/IP packet to the internet, is my MAC address transmitted as source or the MAC address of the router is transmitted.

I use ethernet to connect devices on local network. I use my own MAC addresses which are unique inside the network but are re-used in other networks. Is this any cause of concern.

Like there are private IP ranges of 192.168.x.x and 10.x.x.x, is there a range of private MAC addresses.

Click to expand...

In Norml cases , Mac address is burned on the ROM of your NIC , and it's unique 6 bytes address , half of it identify the Manufacture itself (unique) , and the other half is given by The Manufacture (unique number) so the whole Mac for certain NIC can't be assigned to another NIC .

Abu Ammora said:

techie said:

When I am behind a router, and send some TCP/IP packet to the internet, is my MAC address transmitted as source or the MAC address of the router is transmitted.

I use ethernet to connect devices on local network. I use my own MAC addresses which are unique inside the network but are re-used in other networks. Is this any cause of concern.

Like there are private IP ranges of 192.168.x.x and 10.x.x.x, is there a range of private MAC addresses.

Click to expand...

In Norml cases , Mac address is burned on the ROM of your NIC , and it's unique 6 bytes address , half of it identify the Manufacture itself (unique) , and the other half is given by The Manufacture (unique number) so the whole Mac for certain NIC can't be assigned to another NIC .

Click to expand...

But it can. Just google for "macmakeup" and you'll see how easy it is.