Hi, i used the rasm51 assembler to compile the source, it can be downloaded at:
http://www.rigelcorp.com/8051/rasm51.zip

Witch hardware to use is up to you. (PPP, SLIP or Ethernet)
I have only tested the stack with SLIP but i cant see why it should'nt work with ethernet or ppp if the low level drivers are correctly written. But if you want minimal hardware use slip, the only thing you would need then is a serial cable between your 8051 board and a PC. I have interuppt driven slip drivers to use with the stack, i can send it to you if you want.