For many moons, I have walked alone through the dark valley of unfinished or badly implemented embedded IP stacks. Then, lo, I found the Lantronix Xport:
This small unit is mainly a serial-to-Ethernet converter. I had heard of the Xport before, and was rather skeptical given my continuous failure to find an embedded IP solution that 1) DHCP's a gateway address properly and 2) puts the right MAC address on UDP packets heading out beyond the netmask.
But then I found something else, the Comfile CUBLOC which is like a BASIC Stamp but it has on-board A/D as well as other cool stuff:
You can see the CUBLOC is based on the Amtel MEGA 128. It is programmed in a very Stamp-like BASIC. The Comfile website has lots of example code, including code dealing directly with the Xport. Comfile also sells a Xport Dongle which makes interfacing the 3.3V Xport with 12V RS-232 a snap. Well, except you need to provide +5V on its DB9 pin 9 connector.
On the Yahoo Xport Group I asked if the Xport could really DHCP a gateway address and put the right MAC on Internet-bound UDP packets, and lo and behold a Lantronix engineer answered and said yes.
So I picked up an Xport, soldered it into the Dongle, made appropriate RS-232 cables, set up the Xport through the serial connection, and then I had a serial connection over a specific UDP port between my PC and my Internet server. Success finally!