DCDev - Connect to Your DC
Get a Dreamcast
Get a DC Coders Cable or a Broadband Adapter
Use the cable or the adapter to connect the Dreamcast to your PC
If you have a Coders Cable, download the DCLoad Serial Kit; if you have a Broadband Adapter, download DCLoad IP Kit
For the sake of this tutorial I'm going to use a broadband adapter, so burn the dc-load ip image you downloaded to a CD via DiscJuggler.
Insert the CD into your Dreamcast and switch your Dreamcast on
Once running you should see some text displayed. The third line on the screen is the DC's mac address. We'll be using this later.
I have an ethernet and wifi card in my computer so I'm going to use ARP and map the dreamcast to a local IP on my ethernet card. Set the IP address on your computer's card to 10.0.0.10
Then run in admin mode: arp -s 10.0.0.11 yourdcmacaddress 10.0.0.10. This maps the DC to IP address 10.0.0.11 on the 10.0.0.10 interface.
You now should be able to ping the DC by running: ping 10.0.0.11
Ripped from http://www.dcdev.org/howtos/connect-to-dc.htm