Why you would want to do this:
You've moved to broadband and you don't have a dial-up account to use with the dreamcast anymore, but you still want to be able to connect to the internet with it.
This guide is for you.
You don't have a coder's cable, broadband adapter, or LAN adapter for your dreamcast, and you want to be able to communicate with it.
At the moment, this guide is not the guide for you. There may be a way to set this up, but for the moment, the existing file transfer stuff does not support this.

Quick note for unix newbies: The methods to do this involve using the command line unix interface of Mac OS X extensively. It doesn't assume all kinds of outlandish unix knowledge, but it doesn't go into ultra-fine detail concerning every step. I say things like "run xxx" and I expect that you'll know how to open up a terminal and run it. When I say to edit a file, I'm not going to describe the specifics of how to edit it. Many of the files can be editted in TextEdit, but a few are going to be priviledged, and hence you're going to have to either edit them using "sudo emacs filename", "sudo vi filename", or by using "sudo chmod" to change the permissions on the file, edit it using whatever you want, and then change the permissions back again. So you need to know how to use at least one standard unix editor or chmod. You'll also need to know your password so that you can use sudo, which is a program which lets you do things as superuser which you can't do while wearing your normal user hat.

What you need:
Hardware:
Dreamcast
Some newer Dreamcast models may not work with this method because there were apparently some changes to the modem in some of the later models which requires that the standard phone line voltage be present to dial. The Dreamcast Modem Line Voltage Simulator may help with this if you build one. I don't know diddly about that, however.
Any mac with OS X and a free modem port
Software:
OS X. These instructions will definitely not work with Classic Mac OSes. I would advise 10.1 or greater, and there's some chance that UUCP may require 10.2, but I'm not certain. If these instructions work for you with a version number less than 10.2, please let me know so I can update this section. (Email address at bottom).
One of:
Minicom - easiest to install this from fink, so you should probably install fink first. Gcc can probably also suffice.

cu - You can either get this by building UUCP (for which you will need the developer's tools) or by downloading http://www.asyserver.com/~kirwin/cu.tar and copying cu into /usr/bin.
We're establishing a PPP connection here, so we're not going to be using the real UUCP stuff, but rather just the dialer which comes with it, cu. OS X doesn't seem to have a command-line dialer or other normal telecommunications program. There may be some other graphical one which could do this job that I'm just unfamiliar with, however. So instead we need either cu or minicom.

Set up:
Connection:
Plug one end of an ordinary phone cord into the back of your Dreamcast and the other into your modem.

Dreamcast:
In the web browser of your choice, go diddle with the modem settings. Turn blind dialing on. There will not be a dial tone, so if blind dialing is not on, it will not dial. The other settings you can probably leave alone, but if you want to precisely emulate my set-up, I blanked out the username and password and changed the dialing number to "1". It's just a dummy number designed to be short, but still have some number there since I think that some games check for the presence of a number before dialing.

Mac:
Install either UUCP or minicom. Fink and instructions on how to use it can be found at http://fink.sourceforge.net/ Instructions on compiling UUCP on the mac can be found at http://www.afp548.com/Articles/Jaguar/modem-1.html If you're downloading it and copying it you shouldn't need any additional instructions.
Also, set your computer up to share your internet connection by opening up the System Preferences, going to Sharing, choosing the Internet tab and clicking the "Start" button.
PPP: Set up a /etc/ppp/options which looks like:
debug
default-asyncmap
silent
ms-dns XXX.XXX.XXX.XXX
Where XXX.XXX.XXX.XXX is the domain name server address from your mac. This can be found by typing "nslookup blah". The address listed is the DNS server. It'll also tell you that it can't find blah, but it's not supposed to.
Minicom: minicom users need to run "sudo minicom -s". Go into the Serial port setup menu and change the serial device (option A) to "/dev/cu.modem" if it isn't already set to that. You can also boost the Bps setting (option E) to 57600 if you'd like to communicate at the highest possible rate of data. Then exit the menu by pressing enter and select "Save setup as dfl" and then once it is saved as the default, you can exit minicom.
UUCP/cu: No setup needed.

Using the link

1) Tell the Dreamcast to connect. Do this as you usually would.
2) Tell the Mac to answer.
Minicom: "sudo minicom" Once it comes up type ATA and press enter.
cu: "sudo cu -l /dev/cu.modem -s 57600" Once it says "Connected." (which just means that you've connected to the modem) type ATA and press enter.
3) Have pppd take over the connection. Leave cu or minicom running. Do not quit them. Then in another terminal window, launch "sudo pppd -detach /dev/cu.modem 57600 192.168.0.1:192.168.0.2". Once you've gotten this working well, you can stop using "-detach" and pppd will run in the background. If you've set the speed to something other than 57600, you should use that number instead. If your computer is on a private network using the 192.168 class B subnet, then you should choose a different set of addresses. The two addresses are just the addresses for the dreamcast and the computer across that interface. Any of the normal non-internet addresses from 192, 10, etc. should work fine so long as you aren't using them otherwise.
4) You're now connected. You can ping the dreamcast from the computer and should be able to load web pages via the web browser or connect to any official servers for games which might still exist. Woohoo.
5) When you're done, tell the Dreamcast to disconnect and pppd should close on its own. I'm not sure if it will if you simply turn the Dreamcast off. You might have to kill it in that case. Then you can quit out of minicom (ctrl-a x) or cu (type "~." and press enter), and you're done.

Well, there you go.

Using this document on other unixes:
Darwin: Everything should work except for the setting up of the internet sharing. You'll have to figure that one out on your own.
Others: The internet sharing will definitely differ. You'll have to use whatever NAT stuff you might want to. You'll also need to use a different device. I suspect that /dev/modem will work on some, but you'll have to figure out how to get to the modem serial device on your system. Also, you may well have other options than cu and minicom on your system.

Future changes to this document:
This is essentially complete as is, but I may update it with details about how to do this using cygwin on a Windows machine as well as OS X. If I get specific details about how to do this on other OS's I'd be glad to add it if it's not a huge difference.

Contact:
Keith Irwin <kirwin@eos.ncsu.edu>