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Does anyone know how many MAC addresses a cable modem can support? Does the DOCSIS 3 spec allow for more MAC addresses?

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migrated from serverfault.com Feb 8 '10 at 8:57

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5 Answers

The DOCSIS standard says nothing about this. There might be equipment-specific limitations though, so it's hard to say.

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Your cablemodem will typically have one MAC address on each of its interfaces (e.g. one for the cable side and one for the twisted-pair side) - They are usually different (In my experience with the equipment supplied by Cablevision they are always different), but they could theoretically be the same.

I don't know of any brand of modem that supports multiple MACs on an interface, and the ones supplied by CV don't allow you to change the MAC (and here the cable side is locked down by MAC address, so if you changed that you would probably knock yourself off the network).

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I think people are misinterpreting the question. I think you want to know how many devices with separate MAC addresses you can have on the LAN side of the cable modem. Not, "how many MAC addresses does the cable modem itself have." In any case, I don't know.

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This value is controlled by a parameter in the config file which the cable modem downloads each time it boots up. Usually referred to as "Max CPEs allowed" or similar. Your cable company controls this behavior.

The normal practice is for ISPs to set the value to "1" because they don't want residential customers consuming global IP addresses. They can set it higher.

In the past, certain security flaws may have made it possible for end-users to control this configuration value (and any other value), but normally, the end-user doesn't have any way of altering the value.

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I think it depends on your ISP. Usually they give 1 public ip, if you want more you have to pay. Cable modem should support more than one IP connect in a swith or hub, etc.

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Thanks for the info. But I was wondering what the physical limitation of MAC/IP addresses would be on a single cable modem? –  rmwetmore Feb 6 '10 at 21:15
    
I dont think, the modem is only at layer 2 at most...I think –  llazzaro Feb 6 '10 at 21:19
    
@rmwetmore: any such limitation is built into the firmware of a particular model of cable modem; there is no other factor in determining the limit. –  quack quixote Feb 8 '10 at 10:38
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