What do I need to purchase to use RSLinx Classic's Ethernet driver?

You need TCP/IP software, a network card, and a network connection. The TCP/IP software must provide the WINSOCK API version 1.1. The network card must be connected to your network, in some way. Some cards require external transceivers for this connection. When you purchase your card, also specify the type of network that you will be connecting to. There are many Ethernet cards that come with the transceiver built into the card, for thin-net or 10BaseT networks.