I want to study network. But I don't have an access to a sample network (routers, DNS, IP4, IP6 windows linux mixed heterogenous system). Do you know any online network to study (Free as possible). Is it possible to simulate network topologies with a single PC using virtual machine. If so could you guide me?
If you're looking for a networking lab, you should look into PacketLife Community Lab.
GNS3 is a graphical network simulator that allows simulation of complex networks.
If you're looking at learning about routers and networking on that level, it's hard to beat DynaMIPS and Dynagen as a free resource. You still need to get hold of one (or more) IOS images to run on your virtual Cisco routers and some things are hard to do within DynaMIPS.
It doesn't support emulating switch hardware (due to the high amount of very specialised silicon in a Cisco switch), so it's hard to experiment with VLANs (some things can be done using multi-port ethernet cards and doing VLANs there, but some VLAn functionality (private VLANs being one) can only be done in switch IOS).
While you can implement QoS policies, it's not necessarily easy to test them for accuracy within a DynaMIPS set-up.