Motorola Cablemodem Assigning Public IP to Netgear Managed Switch

Hello,

I am using a Netgear 50-port managed switch (The model is GS750e), in which I created a separate 802.1Q VLAN that only has 3 ports (46,47 and 48).

The purpose of that VLAN is to create a "virtual switch" in which I connect a Motorola Cablemodem, to share internet connection to 2 other computers. My ISP allows the cablemodem to assign 2 dynamic public IPs, so that should work (in theory).

I was trying to get this configuration work for 2-3 months (I didn't spend too much time on it recently) and I only was able to get one computer get the public IP (through DHCP).

Today I had an issue with the internet connection, and when I looked at the Cablemodem's configuration webpage, I noticed 2 IPs were assigned, showing me the 2 MAC addresses registered.

After some checking, I found out (to my surprise), that one of the MACs registered in the cablemodem was actually the one from the Netgear Switch (GS750e).

Still not sure how did that happen, but more importantly, how do I prevent that from occurring?

This now explains why the 2nd computer never got its own IP through DHCP (Everytime I rebooted the cablemodem, the first device to register was the Network Switch). 

I know I can potentially solve this by using a separate switch that doesn't have a MAC address, but if I can avoid doing that, it would be much better.

Ideas?

Thanks!