Re: [gentoo-user] dhcpd versus fixed IP addresses

[Michael]

On Monday, 5 October 2020 11:39:44 BST Neil Bothwick wrote:
> On Sun, 4 Oct 2020 21:47:53 -0400, Walter Dnes wrote:
> >   On their support board, the ISP's tech support told me...
> >
> > a) My computer or router needs to be configured to get the IP
> >
> >    automatically by DHCP to get the connection up and running.
> >
> > b) The TC4400 is a cable modem without any router capabilities, so
> >
> >    there are no wifi antennas on the device. (This was in response
> >    to my question about turning off wifi.)
> >
> >   A Google search turned up a default config webpage and userID and
> >
> > password for the TC4400.  If that doesn't work, I'll look into getting
> > a NAT-ing router that can get a dhcp IP address.  The plan would be...

If the TC4400 is a modem only, then I expect it will act as a half-bridged 
device.  It will sync with the cable headend (CMTS) to obtain a radio 
frequency range and time slot ("talk-time" or TDMA), then exchange Range-
Requests + Range-Responses to complete the DOCSIS protocol sync and move on to 
the IP layer.

Then it will use dhcp to obtain a WAN IP address, default gateway, time (Time 
of Day) server, and TFTP server addresses.  It will download QoS and other 
configuration information over TFTP from the CMTS server, along with 
encryption keys (BPI), in order to complete its registration and obtain a 
Service IDentifier (SID) from the CMTS.

As I understand it, ISP issued modems come already preconfigured with 
encryption keys and hardcoded MACs and they are the only modems which will be 
allowed to register with the ISP's CMTS.  If this is a 3rd party modem, then 
perhaps its MAC address will be used to register and exchange keys with the 
CMTS, but I would think either this MAC address will need to be communicated 
in advance with the ISP so it can be added to the provisioning servers, or an 
ISP obtained MAC address will need to be configured on the modem.  There may 
be variations in this theme including PPPoE authentication via RADIUS servers 
at the ISP end, but I'm not sure this is common practice.

Either way, the modem operating in a half-bridged mode will pass on the WAN IP 
address to the LAN port transparently.  The PC or router at the LAN port will 
need to either know what this bridged WAN IP address is and have it configured 
as a static IP address on its NIC so it can use it, or use DHCP to obtain it.

The above reflects my loose understanding of how this works, or is meant to 
work with a cable modem without routing capabilities.


> I have a Virgin cablemodem that I have switched into modem-only mode, so
> it behaves similarly to your device, and plugged it into the router. It
> gives a setup like you want, except I chose to use DHCP for simplicity of
> administration.

In the old days virgin modems were simple modems only, not routers.  They had 
a GUI with minimal ability to do much, other than check what the WAN IP 
address allocated by the CMTS was.  I suppose these days they're all in one + 
WiFi, with more configuration options.