Create the file /etc/hostname.rl1 with the following content:
inet 10.10.10.1 255.255.255.0 NONE
To activate this interface immediately you would have to type ifconfig rl1 10.10.10.1 netmask 255.255.255.0, otherwise you will have to reboot the machine or run sh /etc/netstart. You can obviously choose a different IP address if you would like, just make sure you choose one that is from a private IP range so it is not accessible from the Internet.
Usually you would now also configure the files/etc/mygate, /etc/hosts and/etc/resolv.conf but we will refrain from editing these now.