Network simulation software

[WakeBdr]

Is there any software out there that will let you simulate a network? When I say simulate I mean set one up within the software and simulate traffic through it to let you see how it would function should you actually build it.

 

[Daniel]

Thats a interesting concept though, not even sure if it exists, but it would been cool to see what effect would happen if you added x users to x speed network without upgrading and stuff.