PPP IP Extension / Half Bridge Mode

What is it? How does it work?

Briefly: PPP IP Extension is a feature of some DSL modems which uses the modem to perform the PPP connection and authentication, assign the internet address to a single LAN client via DHCP and route all inbound traffic to that LAN Client.

Caveats

  • Modem doesn’t get WAN IP until PPP connection is established. LAN Clients get private IP (192.168.1.2) upon modem’s boot up, some modems (SmartRG SR505) have been observed to not reset the ethernet connection upon establishing PPP. This means that the LAN client will keep the private (192) IP until it manually is released and renewed after the connection is established.
    • Workaround: Use Static IP, if you subscribe to a static IP address you can set this static IP in your LAN client (router/PC) and not have to manually renew after PPP session establishes.
  • Inbound connections don’t seem to establish to the client unless a static IP is set
    • Users who do not host services on their connection will not be affected by this.
    • Workaround: Subscribe to a Static IP.

How does this actually work?

This mode of operation in essence is PPPoE mode, which has a LAN DHCP pool of it’s WAN IP and DMZ set for that same IP.

In what situations would I use PPP IP Extension?

This mode of operation is not recommended for most cases.
Recommended is bridge mode
In cases where your router doesn’t handle PPPoE connections properly, it is recommended to upgrade/replace your router.
If your router is not something easily replaceable, it is recommended to utilize PPPoE mode and if necessary add your router to DMZ.
PPP IP Extension should be a last resort.

Other Observations:

Client IP Information received by DHCP:
IP: <WAN IP Provided by ISP> (Example: 24.24.24.5)
Subnet: 255.255.255.0
Default Gateway: <Same as IP> (Example: 24.24.24.5)

Traces from the LAN machine to external hosts indicate the first hop as the Modem/Router as it’s internal address (Example: 192.168.1.1)

External traces to the assigned internet address do not show the modem/router as a hop.

It has been observed that it doesn’t matter what the Subnet or Default Gateway is set as on the client machine. The modem’s NAT translates all packets in on the LAN interface and routes them properly through the PPP connection to it’s own gateway.

See Also: 1:1 NAT, DMZ, Public IP Assignment.

US Robotics Wireless MaxG Modem Guide has a brief description of PPP IP Extension

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