Weird Home Network issue

Hi Guys,

I wasn't sure how to describe my issue in the subject. So I apologize for it being so vague.

Im planning to start building a home lab adding a firewall and a layer 3 switch to be able to create separate VLANS and a VPN etc. But before I could even get started I came across a weird issue, and just want to make sure it does not cause my issues.

Basically I live in an apartment.

I have 2.5 Fiber internet. A thin cable comes from my wall and then connects to a box that coverts that into a fiber cable. Then the fiber cable plugs into a fiber modem. It seems the fiber model is set to bridged mode by default. As when I plug into it with my laptop I'm getting a public IP, as does my main WiFi mesh AP.

The modem has 5 ports on the back.

1 X 10G

4x 2.5G

I plug my main WiFi 7 mesh AP into the 10G port and get great speeds overall through WiFi. When I plug my laptop directly into one of the ports on the main wired AP it's obviously even faster and I'm getting the full 2500 speed as advertised by my ISP.

Then the other day I randomly noticed that in my bedroom and office room there are two lan port outlets on the wall in each room. Just for the fun of it, I plugged in a lan cable and plugged it into my laptop. And boom it worked and it was just as fast as the wired connection would be if i plugged directly into my wired main AP. I checked the IP address, and somehow on the same LAN network (As I'm getting a private IP in the same LAN as the rest of my stuff).

I double checked my fiber modem and there is literally only the Fiber cable, the power cable and 1 RJ45 lan cable plugged into it which goes directly into my main AP as mentioned above. So I'm just wondering how is that even possible? Shouldn't the cables in my bedroom and office somehow be connected to the fiber modem to be on the same network?

My only guess is that somewhere in my apartment building there is a big box which houses all the apartments on my floors cables. And somehow there is a switch there linking the fiber connection in my living room with the 2 Ethernet ports in my bedroom and office room. Is anyone has any ideas please let me know.

That still leaves me wondering how are the other 2 wall LAN connections getting the same 2.5 speeds as when plugging in to the fiber modem, when they are NOT connected to the fiber.

Another weird thing I noticed was that when I check my network settings on any device connected via WiFi or wired into the main AP or any of the 2 LAN ports the subnet is showing as 255.255.252.0 which is a /22, but in my main AP (Which handles the DHCP and subnet etc) is set to the standard 255.255.255.0/24.

No idea whats going on, if anyone has and ideas please do let me know.

Thanks in advance!