Ubuntu dhcp-server how to prevent assignment of someone else's ip?

Question

DHCP

The server distributes the Internet, poppies and the IPs they need are registered in the config.
We add to the network the left computer with a manually entered address, which is already in use on the network - it has the Internet (while the desired computer is turned off at this time).
We turn on the computer to which this address belongs - it does not have the Internet.
How to fix this?

Answer 1

Enable DHCP snooping on switches as a minimum, and for convenience, it is also desirable to enable option 82 on DHCP.

Answer 2

We turn on the computer to which this address belongs - it does not have the Internet.
How to fix this?

IP source guard (core functionality) + DHCP Snooping (address match table), all on the switch.