In order to use TCP/IP over a serial line, you only need a null modem. Here's how the pins are connected on the cable I use:
2 <---> 3 3 <---> 2 4 <---> 5 5 <---> 4 6 <---> 20 7 <---> 7 8 <---> 20 20 <---> 6+8
This cable has proved to work with SLIP and PPP, as well as for using the other side as simple dumb terminal. The devices to use are: