IP Passthrough, simply put, makes the router/modem pass the IP assigned from the ISP to the attached downstream device.
It can be using DHCP to pass the IP address(and DNS server) that has been assigned to a PPP interface by an ISP, to another device running a DHCP client. And also providing a static public address directly to one attached network client.
In IP Passthrough mode, the configured cellular interface do not use the routing code at all (except for some special cases) when handling received IP packets. Packets received for the PPP interface are delivered directly to the specified Ethernet interface. Similarly, packets received for the specified Ethernet interface.
Worth noting is that only one device can get the WAN IP address from the ISP. You may ask that what about when multiple devices are connected to the router, who will get the WAN IP from the ISP? In this case, you can configure the specific MAC address, so it will assign the WAN IP address to the device plugged to LAN port with the fixed MAC address. In IP Passthrough mode, the router can’t work with Link Backup, and the LAN port should be in the type of Static.
What do we need the IP Passthrough for?
Once the device has a public IP address, it can be accessed from the external port via the Internet, so as to be convenient for remote access from another site. But what if you only buy one public IP from the ISP, and you want to access the downstream device?
You may consider DNAT/ port mapping/ port forwarding, but what if there are so many service to be provided, or you don’t know which service ports are already occupied by the router itself, IP Passthrough will just allow you to use the public IP to access the device in the private subnet even though it doesn’t have the PPP dialup functionality. It also save you a lot of effort to configure the more complicated DNAT/ port mapping/ port forwarding configuration. As for the router, it’s kind of a bridge to the downstream device.
Users sometimes state that they need ADSL/Broadband CPE that can be configured or placed into a Bridged Mode where they are putting other CPE behind the ADSL/Broadband CPE. Many times these customers can be better served with a configuration known as IP Passthrough.