|You can fit a network card (that's what an ethernet cable plugs into) into a spare PCI slot inside the system case, but Windows 98 may have a problem detecting it without a third-party driver (Windows 98 does not have as many built-in drivers as later Windows versions, and very few network cards are supplied with a driver these days).|
it's a case of fitting one and see if windows 98 recognises it.
You can also buy a USB card that fits into a PCI slot (drivers may be less of a problem with a USB card compared to a network card).
But you really need a minimum of Windows XP to minimise driver problems when installing new hardware devices. Very few device manufacturers support Windows 98 now.