Telnet is TCP/IP traffic, to use this you would need a network card or null-modem w/ a SLIP/PPP connection. Both require a TCP stack Miami, AmiNET, etc. Nothing is "built-in"...
The easiest and fastest option I found... 3Com EtherLink III PCMCIA (3C589) Network Adapter on Ebay ~$9, and MiamiTCP software. Was pretty easy if your familiar with internet routing and network configuration.