In order to get your Ethernet LAN working for TCP/IP, you need some way of associating each machine's Ethernet card (and thus its hardware address) with an IP address. Fortunately, there's an easy way to associate an IP address with an Ethernet interface under Linux. In fact, if you are currently using Ethernet with Linux, your distribution's system initialization scripts very likely have a command in them that looks something like this:

ifconfig eth0 192.168.1.1 broadcast 192.168.1.255 netmask 255.255.255.0

Above, the ifconfig command is used to associate eth0 (and thus eth0's hardware address) with the 192.168.1.1 IP address. In addition, various other IP-related information is specified, including a broadcast address (192.168.1.255) and a netmask (255.255.255.0). When this command completes, your eth0 interface will be enabled and have an associated IP address.