DPC3939B in true bridge mode, with single static IP?

Hello Kremlar and welcome,

Please see my comments/answers to your questions below:

1 - Is it possible to configure this DPC3939B gateway to be in true bridge mode and keep our single static IP?  With every other gateway I've used this is not only possible, but desired for our needs.  However, I'm reading some info on these forums that makes me think this is not possible with this device.

It is nor possible to have any industry standard Internet gateway operate correctly using a sanctioned static IP address while being in true bridge mode. This is due to true bridge mode disables ALL internal routing and therefore this includes the static IP addressing routing with the Internet gateway WAN programmed static IP gateway address. True bridge Mode ONLY  provides the pass through of the Internet gateways WAN address, but again, does not provide any routing between the static IP routable address and the gateway address wrt the dedicated subnet mask. This is not a DPC3939B restricted device issue because it is an industry standard operational issue plain and simple from a technical operation. 

2 - Our current publc IP is xx.xx.xx.206.  Once placed into true bridge mode, or pseudo bridge mode if true bridge mode will not allow me to keep my static IP, how do I know what to use for a gateway and subnet?  I don't see these anywhere on the DPC3939B's web interface.  Actually, it does show a WAN Default Gateway Address (IPv4) but it seems to be in a completely different subnet as our public IP so I don't believe that is correct.

All industry standard Internet gateways have either WAN DHCP or static IP addresses. For a WAN DHCP IP address is could be something like 75.216.32.98 with subnet mask 255.255.255..0 or a static IP 75.216.32.98 with 255.255.255.252/248/240 (for single/5 block/or 13 block) subnet mask examples. I believe the DPC3939B shows the WAN address, gateway, and subnet mask either in the software or hardware display function within the user interface. Comcast provides a dual stack IPV4 and IPV6 for all of its Internet Gateways. If your internal network device can use IPV6 in stateless mode i.e. DHCP mode then this will work fine. However, if you are using IPV4 with a sanctioned IPV4 static IP address, then your internal network should be able to handle this no problem just like many other Comcast business customers. 

So, from what I gather from your initial post, the ONLY thing you must concern yourself with is make absolutely sure that your Server device's network interface is programmed correctly with your static IP routable, gateway, and subnet mask that was provided to you and good practice is setting your DNS pri & sec addresses, too.  You should test this by making sure your Server is directly pluugged into one of the DPC3939B LanPorts 1-4, then jump onto the intenet and if you can ping your static.IP.routable.address then you are good to go. If not then you have a Server network interface staticIP programming error, cable interconnect error, etc. If all is good to go here, then you should simply log into the DPC3939C with a Enet cabled computer connected to any of the free LanPorts 1-4 on back, then bring up a browser use 10.1.10.1, and disable the internal LAN DHCP server to avoid any conflicts and use your Server's DHCP server.  We have many other Comcast business customer that use the aforemention networking very successfully. Hopefully you will become another. 

FYI, the rep seemed to be clear about it.  Said true bridge mode required a static IP (even though I told her the forum sticky said otherwise) and had to be escalated to tier 2 and they would do it within 24-48 hours - which seemed crazy to me.  That's something I should be able to call in and get done at that time to avoid random downtime, as opposted to rushing to test/tune my router whenever they decide to reconfigure the gateway within that span of time.

She said pseduo bridge mode could be enabled myself by disabling DHCP.

I notice the DPC3939B has an option in the configuration page that enables/disables "bridge mode".  Is that for true bridge mode, or for pseudo bridge mode, or something else??  

That Bridge Mode is the True Bridge Mode, but I was unaware that is enabled for customer implementation. Pseudo Bridge mode is simply disabling the DPC LAN DHCP Server.

I hear you loud and clear.  Sometimes osme folks get mixed up between Psuedo Bridge Mode, which is disabling the Comcast Gateway DHCP Server versus True Bridge Mode, which means that the Comcast Gateway will simply pass the WAN address for another device to use, but for a Comcast static IP address in True Bridge Mode, the routing for the staticIP routable address is totally disabled.  So this is why, having a Comcast static IP within a Coomcast Gateway in True Bridge Mode just doesn't work. 

Psuedo Bridge Mode should work just fine for you, too.

Funny, because a Comcast rep I spoke with earlier today says true bridge mode REQUIRES a static IP address.

Lots of misinformation out there, but the bottom line is we need a static IP address, need to use our own router apparently, and don't want the Comcast gateway blocking any traffic coming in.  I'm hoping the 'psuedo' bridge mode will do what we need.

It seems like such a simple need - what happened to the days of straight cable modems?