IP Passthrough (IPPT) vs Bridge Mode

IPPT and Bridge Mode Overview

Bridge Mode

The modem acts purely as a Layer 2 Ethernet bridge. It transparently forwards Ethernet frames (or 802.1Q VLANs) between its WAN (cellular) interface and its LAN ports, no IP stack on the device itself.

• Layer 2 only: the gateway bridges VLAN(s) or untagged Ethernet from its radio to its LAN ports.

• Router must speak carrier’s PPPoE or DHCP‑VLAN: often you must configure 802.1Q tagging or PPPoE credentials on your router’s WAN port just as if it were cabled directly into the tower.

• Carrier’s DHCP/PPPoE interactions are exposed directly to your router.

IP Passthrough (IPPT)

The modem still terminates the cellular session and gets assigned a public IP by the carrier, but then uses DHCP (or static hand‑off) to hand that single public IPv4 (and/or delegated IPv6 prefix) to a designated LAN port or device. The modem does not NAT or route beyond that one hand‑off.

• Layer 3 hand‑off: ISP modem terminates the cellular session, then runs a mini‑DHCP server that hands the carrier‑assigned IP to your router.

• Your router sees a normal DHCP WAN just like a cable modem.

Routing L3 Workflow

Bridge Mode

1. Router opens WAN interface → uses PPPoE or 802.1Q VLAN tagging (as required by carrier).
2. Carrier DHCP/PPPoE replies → assigns public IPv4 & gateway.
3. Router → NAT + routes all LAN traffic.

The ISP modem stays invisible at Layer 3, it simply passes frames back and forth.

IP Passthrough

1. ISP Modem runs its own DHCP client on the cellular interface → gets public IPv4 & IPv6 PDN.
2. ISP modem publishes that single IPv4 on a LAN port via its own mini‑DHCP server.
3. Router on that port obtains the same public IPv4 via DHCP → uses it as its WAN IP.
4. Router → NAT + routes all LAN traffic.

[Internet]
    ↓
[Cellular Core → eNodeB/gNodeB]
    ↓
[ISP Modem WAN radio] ── DHCP/PDN → gets public IP & IPv6 prefix
    ↓  (internal DHCP client)
[ISP Modem hands off WAN IP via special LAN port]
    ↓
[Router WAN port] ── DHCP client → grabs that same public IP
    ↓
[Router LAN port] → your LAN (private subnets)Code language: PHP (php)

Modem’s Role

• The cellular modem terminates the 4G/5G session, gets one public IPv4 (and/or an IPv6 prefix) from the carrier’s Packet Gateway.
• It then runs a tiny DHCP server on one of its LAN ports that contains exactly that one public IPv4 address.

Router’s WAN Interface

• Your downstream router’s WAN port is plugged into that designated LAN port on the modem.
• It brings up its WAN interface as a DHCP client, pulls in the public IPv4, and (if enabled) grabs the delegated IPv6 prefix.

NAT Location

• All NAT is done on the router.
  • On the router you mark the WAN interface as ip nat outside (or equivalent)
  • You mark your LAN-facing interface(s) as ip nat inside
  • Then you apply an overload (PAT) rule so that all your private‑LAN traffic is source‑NAT’d to the single public IPv4 your router has.
• The modem does no NAT routing for your LAN clients, it merely “passes through” that public IP.

The ISP modem only handles the cellular attachment; routing/NAT rests entirely on your router.