Software for simulating a network on single PC(without having network).i need it for programming purpose.?

i want a software which can simulate a network on my PC. i want to learn some network programming .but i dont have any network.is there any software which can simulate a network on which i can make and test my network appplications.

Software for simulating a network on single PC(without having network).i need it for programming purpose.?
You can use ns-2, which is an open source... Look at:


http://www.isi.edu/nsnam/ns/
Reply:Not that I know of, however if all you need to do is test port functionality, you could just tell your app to connect to 127.0.0.1 ...granted, that doesn't simulate firewalls or any of that jazz.