Cable Systems

What you see when you purchase High-Speed Internet access via Cable Modem  .  .  .  is the cable modem.  What you don't see, is that the cable company had to change their infrastructure, including right outside your home, to convert their system from Video only . . . to Video & Data (Internet).  Here is a setup that includes both Satellite and Cable - of course few would get both, but this diagram is for instructional purposes only. 

Cable Modem

Surprisingly, this device is transparent to you, for any trace-routes you might issue.  It acts as a bridge.  Of course, it is also converting the RF IP MPEG frames signal to an IP Ethernet signal.

CMTS device (Cable Modem Termination System)

The CMTS provides many of the same functions provided by the DSLAM (DSL ATM Multiplexor) in a DSL system.  It combines (multiplexes) signals from multiple customers into one Internet pipe.  This device can act as a bridge or router, depending on how your provider has it configured.  If it is performing routing, it will show up in any trace-routes you might issue.  This is a small computer, that enables cable modems to send and receive packets over the Internet. Downstream, it converts IP packets from the Internet into MPEG frames and transmits them to the cable modems via an RF signal. Upstream (coming from the cable modem to the Internet) is does the reverse.

The CMTS performs packet format conversion and DHCP addressing. It may also provide routing, bridging, filtering and traffic shaping. The combiner merges the TV programming feeds with the RF data from the CMTS.   If routing, the CMTS will show up as one hop, and you will see it's address as the private reserved Class A "10" network, in any traceroutes you may perform.  For example, here is a traceroute from my home, into the Cable modem, to

A DOCSIS-compliant CMTS enables customer PCs to dynamically obtain IP addresses by acting as a proxy and forwarding DHCP requests to DHCP servers. A CMTS may provide filtering to protect against theft of service and denial of service attacks or against hackers trying to break into the cable operator's system. It may provide traffic shaping in order to guarantee a specified quality of service (QoS) to selected customers. A CMTS may also provide bridging or routing capabilities.