Does anyone know how many MAC addresses a cable modem can support? Does the DOCSIS 3 spec allow for more MAC addresses?
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).
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.
Does anyone know how many MAC addresses a cable modem can support?
Does the DOCSIS 3 spec allow for more MAC addresses?
MAC addresses are a property of an interface, or socket.
You have 2 such interfaces, or sockets on your cable modem - 1 where the cable from the cable company plugs into, and 1 where you can plug in an Ethernet port.
Each one of these interfaces has a MAC address, and just one MAC address.
This doesn't strictly apply to your cable modem, but some physical interfaces don't have MAC addresses if they are bridge or switch type interfaces that are designed to be invisible on a network.
If you are hoping that you can connect 2 devices to the LAN side of your cable modem using a hub or switch, it won't work. Every cable modem I've experimented with will only talk to the first device that responds on the LAN port. If you want to share the cable modem connection, this device needs to be a router, or a device that has a built in router (there are "cable modem routers" that consist of both the modem and router part in 1 chassis).