Why does Sagemcom Gateway F@ST 3864OP not list all DHCP clients?

[quantoid]

I'm having problems with my modem on NBN, and while trying to get it to work I noticed that the list of DHCP clients seems to be incomplete and often shrinks and grows at random. I also get warnings on some computers that other computers are using the same IP address, and the list of DHCP clients shown in the modem's admin interface (at 192.168.0.1) shows duplicates. 

Most of my devices connect via an Airport Extreme set up in bridge mode, so it delegates the DHCP leasing to the modem/router. 

So I have two questions:

1. Why is the list of DHCP leases in "Advanced Settings > Device Info > DHCP" incomplete and fluctuating?

2. How do I stop two devices getting the same IP address?

[quantoid]

cricket...

cricket...

[Guy]

Hi qantoid. It's likely that any discrepancies in the devices showing in the DHCP list are due to the relaying of that information from the third party hardware that you are bridging through (Airport Extreme). This set up is not officially supported by us I'm afraid. You can check to see if there is a difference by connecting the devices directly to the Optus equipment. IP addresses are allocated dynamically by the Sagemcom and we aren't able to provide a solution about changing IP addresses per device I'm afraid.

[quantoid]

The AirPort Extreme is not doing any DHCP, it's configured in bridge mode. The only thing giving out DHCP leases is the Sagemcom router/modem, which is the thing that's also showing inconsistent lists.

The Airport settings here show that DHCP is off:

The list of DHCP clients in Sagemcom's advanced (native admin) interface is missing a whole bunch of devices that I know have a current lease:

The list of DHCP clients shown the Optus-branded simplified "My Nework" page is much longer but appears in a very short scrolling box so I had to hack the HTML to see the full list for this screenshot:

 

This inconsistency makes it really hard to track down what's going on when there are duplicate IP addresses.

[quantoid]

Given both the Optus-branded pages and the advanced pages are running on the same router/modem, and have nothing to do with the Airport, this inconsistency shows there's something odd about how the DHCP leases are reported on the Sagemcom.

[quantoid]

"IP addresses are allocated dynamically by the Sagemcom and we aren't able to provide a solution about changing IP addresses per device I'm afraid."

Thanks but I know how to statically assign an IP address per device, that's not what I'm asking. The problem I'm getting is that multiple devices are being automatically allocated the same IP address by the Sagemcom, which DHCP should not do. I don't want to have to statically assign IP addresses to work around that, I want to fix the problem that's causing the dupes.

[slowlearner]

I was having a lot of trouble using this modem with an airport extreme in bridge mode. It took forever to connect due to sagemcom having a really flaky dhcp. Connections would drop etc. The workaround was to run two DHCP servers one from the sagemcom for wired LAN the other from the airport extreme with the range split between the two. eg sagemcom (wifi disabled) DHCP range from 192.168.0.1 through 100, then airport extreme running 192.168.101 - 254.

This set up is working really well.