Thank God GNS exists! I extensively used GNS for my Cisco career certification exams. I only played with routers in the GNS so far - which was enough to experiment with routing protocols and other relevant stuff for CCNP/CCIE. Loopback interfaces substituted for the actual hosts. However, I wanted to go to next step and actually run small Linux images on Qemu and integrate it in GNS labs.
It turned out that setting up qemu was a piece of cake. Finding a small Linux image for qemu was a bit of a fight, but I found Linux microcore 2.10 image that worked very well with qemu on Windows 7 host OS. And voila! My first GNS lab with 3 hosts, one router and a switch was up and running in no time. Following is a screen shot of the network. I initiated ping to "host3" from "host1", which is visible on host1's console. Host3 was running tcpdump to capture ICMP packets. On router, "show interface stats" shows increase in octets and packets. Instant gratification! :)
(Please click on the image to see the larger version).
This has added a lot of interesting stuff in the "TODO" list, for example, compile my own small linux image, so that I can add a few more tools in there, IPv6 network with hosts, ASA and PIX firewalls in GNS etc. Let's see how it goes..