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What does the following items on my modem (SURFboard Model:SB5101) mean:

Customer S/N: xxx
S/N: xxx


Why are there two S/Ns and two MAC IDs? Thanks!

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up vote 2 down vote accepted

This link explains the MAC addresses: From the link:

HFC = Hybrid Fibre-Coax. This abbreviation refers to the port of the cable modem that connects to the CATV coax cable. Some cable ISPs require you to register with them the HFC MAC address of your cable modem.

Cable modems might have multiple MAC addresses, one for the HFC side (cable TV coax socket), and one for the CPE side (Customer Premises Equipment, the ethernet socket). USB-capable cable modems also have two further MAC addresses: one for the cable modem USB socket, and another for the emulated network interface in the USB driver in the PC. Although it is possible to discover the CPE MAC address(es) of the cable modem, there is no application or procedure that requires them.

I'm unsure about why there are 2 S/Ns, though. I would imagine one means something to a cable company, while the other means something to Motorola.

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Thanks! (1) Where are cable TV coax socket, the ethernet socket, the cable modem USB socket, and the emulated network interface in the USB driver in the PC? Is "the ethernet socket" on the customer's computer? Are the last two also on the customer's local computer? (2) "the emulated network interface in the USB driver in the PC", why is it called "emulated"? Emulating what? – Tim Jul 3 '12 at 17:47

The page that faircy linked to explains why your USB modem has two MAC address.

Here are some technical details to explain it all in a more general context.

A MAC (or Media Access Control) address is a 6-byte unique identifier for networking hardware. Each Network-hardware manufacturer gets its own 3-byte "area code" if you will, and then within the remaining three bytes, assigns unique numbers to each network adapter that they make.

This is supposed to allow each and every network device to be uniquely identifiable (which is required for proper networking). (IP addresses are not sufficient because a network device can have 0 or more IP addresses.) Think about how phone numbers work; if every person has their own number, then they can all be reached. If some people have to share, then they have to use some sort of system within their home so that messages get to the right place (an answering machine is a sort of NAT).

(That said, MAC addresses can be spoofed.)

The reason that your cable modem has two MAC addresses is that it has two network interfaces. Imagine instead of a cable modem, you had a residential gateway. That is, a whole computer that acts as the gateway between your home computers and the Internet at large. That computer would have a network card connected to the Internet and one or more additional cards to connect to the home systems. Each of these cards would have their own MAC address.

The cable modem is the same thing. It has one network interface that connects to the ISP, and another one that connects to your computer/router. Each one has its own MAC address.

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Thanks! When surfing the internet, whose MAC address is exposed to the world, my computer's or the one of modem's facing the outside? – Tim Jul 3 '12 at 17:28
Neither; networking hardware does not have direct access to MAC addresses, that's what IP addresses are for. To stretch the phone analogy, imagine you are in an office. People can contact you with a phone number, but it does not go directly to the phone on your desk; instead it first goes to a central switch-box which may do some security checks and such, then directs the call to the private, individual, internal number associated with you phone. – Synetech Jul 4 '12 at 1:56

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