Why does my Mint PC disrupt the entire network when it goes to sleep?

The network adapter and its driver are worth investigating, especially if the Ethernet connection uses a USB adapter or docking station. Some independently powered adapters can misbehave during suspend and potentially flood the network with pause or other control frames. Run `lspci` and `lsusb` to identify the adapter, then search for known driver issues and test different power-management settings. If possible, try another adapter or connect the PC through a different switch port.

The best way to identify the cause is to capture traffic while the computer is asleep. A packet capture with tcpdump or Wireshark could reveal multicast floods, unusual broadcasts, or another kind of malformed traffic coming from the sleeping machine. Checking the router's logs and client list at the same time may also show whether the Mint PC is trying to act as a DHCP, DNS, proxy, or gateway service.

Try waking the PC instead of shutting it down and see whether the network recovers immediately or stays broken until power-off. After waking it, check `journalctl -p 3` for driver or suspend-related errors. Testing with qBittorrent fully closed is also worthwhile, and an `iperf3` test between two local devices can distinguish a local network problem from normal ISP speed fluctuations.

Wake-on-LAN is a good first thing to test. Temporarily unplug the Ethernet cable while the PC is asleep and see whether the rest of the network immediately recovers. If it does, disable Wake-on-LAN in the BIOS or with the network settings. You can inspect the current setting with `ethtool | grep Wake-on` and disable it with `ethtool -s wol d`. Also check which sleep state is being used with `cat /sys/power/mem_sleep` and review possible wake sources in `/proc/acpi/wakeup`.