By design AT&T gateways do not have a ‘bridged mode’, AT&T REQUIRES the RG (remote gateway/modem) for example: BGW-320. This “guide” was created by IT people to get the BGW-series to behave as if they are in a ‘bridged mode’. BGW-210 / BGW-320 (example below shows BGW-320 using a Sonic Firewall)
If the goal is to get a Static IP to a 3rd party router/firewall [abbreviated ‘3PR/F’] -- the easiest way to use your static IP’s (/29 5 IP range) behind the BGW-320 is via "Public Subnet".
No need to go through "cascaded router" or "IP Passthrough", leave these “OFF”.
In this example, you were provided with the following static IP info from AT&T:
Gateway: 99.44.42.158
Starting/Ending IP range: 99.44.42.153 – 157
Subnet: 255.255.255.248
Setting up the “Public Subnet” -- browse to the BGW-210 admin page (192.168.1.254)
STEP #1: Go to Home Network -> Subnets & DHCP. [you will be required to enter the ‘Device Access Code’ printed on the back panel of the BGW-320]
Under the section "Public Subnet", set "Public Subnet Mode" to "On", "Allow Inbound Traffic" to "On". Enter your AT&T assigned static gateway IP (99.44.42.158), subnet mask (255.255.255.248), start and end address *(99.44.42.153 / 99.44.42.157). *If your goal is simply to have the [3PR/F] with a static IP, we recommend using 99.44.42.153 for both start/end. Make sure the "Primary DHCP Pool" is set to "Public". And ”Cascaded Router Enable" should be "Off" on the same page. Click the Save Changes button.
STEP #2: Go to Firewall -> IP Passthrough and set "Allocation Mode" to “Off” (if not already).
STEP #3: Under Firewall -> Packet Filter, click the ‘Disable Packet Filters’ button, and under Firewall -> Firewall Advanced, disable some settings as you see fit. This assumes you will be using your own router/firewall, which should certainly be the case if you're using a SonicWall.
STEP #4: The BGW-320's setup is complete. Now go to your [3PR/F] WAN interface, enter your purchased static IP (1 of the 5), gateway etc.. (IP assignment Static ), as if your [3PR/F] is directly connected to a bridge mode modem. No need to do any special 1:1 NATs, just the standard outbound NAT rule that the [3PR/F] should create for you by default will work. If you have any special inbound NAT rules (port forwarding etc.), adjust them as needed.
That's it. This is basically a "pseudo bridge" mode, and the best you can do via the BGW-320. Please note that you're still effectively using double NAT; it's just that the first layer of NAT (at the BGW-320 gateway level) is pretty transparent, and all traffic should be allowed through. However, you're still subject to the gateway's 8000 NAT session limit.