Xfinity Modem/Router Handing Out Strange IP Addresses

I have an Xfinity (Comcast) provided Modem/Router combo that's been acting strange recently. It's a DPC3941T manufactured by Cisco.

I have the router set on a 255.0.0.0 subnet with DHCP enabled for the addresses 10.0.0.1/24 and Wi-Fi is disabled. I have a Ubiquiti Unifi AP Lite for the Wi-Fi access throughout my home.

But what I have noticed recently is that the router seems to be handing out IP's on a completely different subnet. And all of these other IP's have no similarity other than the 10 at the start. Here is a screencap of the offending devices.

None of the affected devices have restricted access. Speeds are the same as usual and I don't really have a way to determine if they are accessible from other subnets however, chromecasts still work.

Any ideas as to how to fix this? Or should I just let it be since nothing seems to be affected?

Thanks

1 Answer

Those addresses are all in the same network if your network mask is 255.0.0.0.

Apparently you are unfamiliar with IPv4 subnetting. An IPv4 address is a 32-bit binary number. The dotted-decimal notation that you see is simply to make it easier for humans to read. To determine the network of an IPv4 address you simply perform a logical AND with the address and mask. If the result is the same among addresses, they are on the same network.

For example, one of the addresses about which you are concerned is 10.5.83.205, but you are not concerned about the following address, 10.0.0.220. The mask for both addresses is 255.0.0.0. If you AND both addresses with the mask, you end up with the same network, so both addresses are on the same network:

10.5.83.205 is 00001010000001010101001111001101
255.0.0.0 is 11111111000000000000000000000000 AND ================================ 00001010000000000000000000000000 = 10.0.0.0
10.0.0.220 is 00001010000000000000000011011100
255.0.0.0 is 11111111000000000000000000000000 AND ================================ 00001010000000000000000000000000 = 10.0.0.0