You not getting a response can mean any number of things. Be patient.
To answer your question, There are two popular TCP/IP stacks for low resource controllers. They are uIP and lwIP. Since you mentioned 89c51, I am assuming its from ATMEL so I guess you won't be able to fit the fulll stack within the microcontroller itself, you may have to use some sort of External RAM.
Also you have to understand that these tcp/ip stacks are there for functionality and not for performance. At most they can handle one connection in low end controllers.
If you have something specific in mind, just ask.