# Networking – Is it possible that a lower level network can provide IP addresses for the higher level network? And how to prevent that

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Sorry for my obscure title, I don't have much knowledge about networking. So today I got a really strange problem with my network, it's like this:

My network:
From Internet → my Wi-Fi router → "black box" ("black box" means that one cable get out of my Wi-Fi router from the LAN ports group, go somewhere and I don't have access to that network, I don't know what's going on there)

My PC and cell phone connect to my Wi-Fi router (so the higher level network in the title means this network and the lower level network means the "black box").

Normally my devices have IP addresses like this:

• My Wi-Fi router: 192.168.197.1
• My PC: 192.168.197.101
• My cell phone: 192.168.197.102

Today they changed, and my phone IP address became 192.168.21.106, and my PC IP address changed to 192.168.21.107.

Because I think that somehow the "black box" part conflict with my network, the 192.168.21.x came from the "black box" and it provides IP addresses for my devices, so I decided to try set a static IP address for my PC and my phone back with its normal IP address (kind of force they choose one specific IP address) and it works, I can connect to the Internet again.

So my question is "is it possible that a lower level network can provide IP addresses for the higher level network?". I feel really strange because I don't think a "slave" network can affect/sabotage "master" network like this so it leads to another question "how do I make sure that my network is a real master-slave?, or make sure the higher level network never gets IP addresses from anywhere else, but the primary router?".

Another thing I should add, I call it "black box", because I think my network should work with whatever network setup or happen in the "black box", so I don't like the solution like turn off/disable DHCP on "Black box" too much.

EDIT: I got two evidences tell me that the "black box" is the problem. One is that when I unplug the cable which connect to the "black box", 192.168.21.x is gone, everything back to normal, and two is that when I set my PC to the IP address 192.168.21.107 (IP address from "black box") I can connect to 192.168.21.1, and it's a management website of a Wi-Fi router (not mine).