Did the machine not detect the ethernet card when it was booted?
How many ethernet cards have you got? If one, then just go to the redhat-network-gui tool and you should be able to see the card. Most cards are pretty much detectable now.
If not, and you have a linux driver, you'll need to mount the floppy so that you can access it:
mount /dev/fd0 /mnt
You should then be able to access the driver via command line. If the driver is an rpm, then just do:
rpm -ivh <driver>.rpm
Check the README file you should have for the driver and all should be straighforwards.