Hi:
What is the best software for simulating network with a computer?
What is diffrence between vmware server and vmware workstation and virtual server?
If I want to have a network with a computer ,which software of them is necessary?

I'm not a VMWare expert but I do use VMWare server for testing applications in a controlled environment. VMWare server is free, workstation is not, MS was giving away Virtual Server at one time but I'm not sure if they still are.
If you want to simulate a network on a single PC then you're best bet is probably VMWare Server but there are a few things to keep in mind... especially if you are setting up multiple windows machines... the following is true for either VMWare or Virtual Server
1. You will need a legal copy for every virtual machine you configure. Installing a copy of Windows XP on a virtual machine is no different from setting it up on your desktop... it will still need to be activated.
2. You will need a LOT of RAM if you're going to setup more than 2 virtual machines.
I have a Windows Server 2003 VM and a windows XP VM that I run on my XP pro desktop. I have 2 gig of ram and I have only a few hundred K free when I'm running both VM's at the same time. Even with a core 2 duo it's still sluggish.