DOCSIS is the standard for IP networking over cable TV infrastructure. An open source modem can&#x27;t be built because there&#x27;s a huge certification &#x2F; documentation fee from CableLabs and part of the requirements involve the cable carrier being able to control&#x2F;update the modem at their whim.<p>Putting your router behind the DOCSIS modem lets you firewall the modem the same way you&#x27;d firewall the Internet at large - that is, an attacker who compromises the modem wins the ability to specifically monitor your traffic, but does not immediately gain free access to your local network.