boot old computer from ethernet with laptop as server

Solution 1:

This is possible if the BIOS of the old computer supports Preboot eXecution Environment (PXE) as PXE client.

Since you don't have an operating system on the old computer, if it supports PXE then on boot the BIOS will automatically search as a last resort for a PXE server on the network (Ethernet only).

If you don't see it searching for the PXE server, then the computer doesn't support PXE. You could also search its BIOS for a PXE option that you can enable (if one exists).

If you do see it, you will need to setup the laptop as a PXE server, which needs some technical ability.

For more information see the article preboot execution environment (PXE).