I recently set up a new internet connection at home and my modem didn't support bridge mode. Instead I had to use an option called "IP Passthrough". Here's some details on that modem configuration. I've never heard of IP Passthrough before, and there's not much technical information about it in particular.

Here's the physical details of my modem:

[WAN]  - Connected fiber or DSL (PPPoE)
[LAN0] - Connected to my router with public IP assigned
[LAN1] - \
[LAN2] - -> LAN 1-3 Still receive DHCP with NAT
[LAN3] - /

So what exactly - in non consumer terminology :) - is "IP Passthrough" doing under the hood to provision my public IP address on my router, while also using the same public IP for itself? Is this Proxy ARP or similar?

Another thing referenced in that reddit thread is "DHCPS-fixed". I've never heard of this either, and it doesn't seem the same as a DHCP Relay.

DHCPS-fixed seems to work best as it allows you to specify the MAC address of the device to pass traffic through to. It is worth mentioning that this is still a DHCP address that your internal device is getting

It'd be great to get a better understanding of these terms and learn how these methods work. Thanks!

Additional documentation link from ATT

  • Now that I think about it, maybe DHCPS-fixed is a reference to "DHCP Server" with some kind of "Fixed" configuration based on the MAC I configured on the modem. – Blaine Nov 18 '20 at 6:47
  • It's well documented, if you know where to look. It's just NAT. It just seems strange because the RG is allowing a device behind it to also use the same address. Anything that isn't associated with the RG is then "passed through" to the inside device. – Ricky Nov 18 '20 at 10:20
  • Thanks @Ricky it's certainly not a standard NAT, right? The host connected to LAN0 has the public IP address of the modem assigned by DHCP. - What would the route table look like on the modem? (Given two devices respond to the same IP, maybe a secondary IP is present?) - What would the NAT rules look like on the modem? (My assumption is this must be done by interface or MAC) – Blaine Nov 18 '20 at 18:41
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer. – Ron Maupin Dec 31 '20 at 4:00

DHCPS-fixed: The WAN IP address will be handed out by the device's DHCP server to the LAN client whose MAC address has been specified. It may be helpful, after the device has obtained a WAN IP address, to cycle the LAN client's interface or reboot it. On disabling this feature, you should reboot the LAN client.

According to this quote modem should acquire external IP and use it to talk to uplink and also will pass that IP to specified LAN client. This means you have DHCP server on modem and one of client's MACs is marked as "external". "external" will get real IP address in DHCP response, but all others will get 192.168.x.x.

In this case modem become transparent to both "external" LAN server and uplink. Yes, it works close to "proxy arp" because modem is proxying uplink to LAN server and vice versa, but it is also mixed with NAT, because LAN server traffic is mixed with other LAN clients traffic and all those live in one NAT table. Not too much differ from usual port forwarding except one LAN client address is forged to look like real IP.

  • No, it's not "proxy arp". Proxy-arp is a real, well documented, thing. – Ricky Nov 18 '20 at 10:21

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