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In a standard home network: ISP <--> Modem <--> Router <-> devices.

There are 2 networks here- LAN and WAN. The router address is 192.168.1.1, the devices are on 192.168.1.1/24 and the WAN address is x.x.x.x. However, my modem has an IP address as well- 192.168.100.1. This is neither in the LAN nor in the WAN. What network is modem on? When I access the admin interface of the modem, how does the router route these requests, since it is neither an internal LAN IP nor an external address.

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It is possible for multiple unrelated IP subnets to coexist on the same physical link. By default they don't interact with each other (there's no L2 separation between them, but the hosts simply aren't aware of the other subnets), although most operating systems can be told about additional subnets on the same link if necessary.

So the modem's management interface is on the 'WAN' network at layer 2, but at the same time it's a fully separate network at layer 3.

(For example, you could also configure your PC to be 192.168.77.1/24 while being connected to the 192.168.1.0/24 LAN. Then you could configure another PC to be 192.168.77.2/24, and they could communicate with each other, but not with the regular 192.168.1.0 subnet.)

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