At this time, Knetstart does not install the drivers for your ethernet card. You need to do that by reading the Ethernet HOWTO. The most important piece of information is the name of the kernel module that will work with your brand of card. Here are a few of the popular cards and their Linux modules:
[Card brand] [Linux driver name] --------------------------------- 3com 10Mbps: 3c509 3com 100Mbps: 3c59x Intel 100Mbps: eepro100 Intel 10Mbps: eepro DLink 100Mbps: via-rhine OvisLink 100Mbps: rtl8139
When you find the right module for your card, add the following line to the file /etc/conf.modules to install the ethernet driver:
eth0 3c509
Replace 3c509 with the name of your card's driver. Save the file, and then run knetstart again. There is a chance that you did everything correctly and if so, it will work. In the future knetstart should be able to do much of this for you.