1. Network Simulator software
Without having physical device you go for certification, Topology design,
analysis, concepts and configuration
CISCO
Packet tracer, Good for beginner (Limited feature support)
CISCO IOU (IOS on UNIX) running on UNIX OS
Others
GNS3 Very good for intermediate and advanceuser using CISCO IOS
NS (open source)
OPNET (proprietary software)
NetSim (proprietary software)

2. Know more about network simulation
http://en.wikipedia.org/wiki/Network_simulation
Know more about network CISCO packet tracer
http://en.wikipedia.org/wiki/Packet_Tracer
Know more about network GNS3
http://en.wikipedia.org/wiki/GNS3
CISCO IOU (CISCO IOS on UNIX)
CISCO proprietary network simulator it requires less resources than GNS3 run ON
UNIX operating system.
FAQ for CISCO IOU
http://evilrouters.net/2011/01/18/cisco-iou-faq/
http://evilrouters.net/2011/01/21/example-cisco-iou-topology/
http://freeccnalabs.com/resources/topology/
Running CISCO IOU on GNS3 (Warning this only tutorial)
Required components
1) CISCO IOU image
2) CISCO IOU image license
3) GNS3
4) Virtual box
5) CISCO IOU compatibleUNIX VM

3. Download(GNS3)
https://pypi.python.org/pypi/gns3-server/1.1
DownloadUNIX Virtualmachine for running CISCO IOU
http://sourceforge.net/projects/gns-3/files/IOU%20VMs/
CISCO IOU License file
Create license using note pad (See below) save it
File name = IOURC.txt
DownloadCISCO IOU images link below
http://fastpiratebay.eu/thepiratebay.se/torrent/9418570/Cisco-IOU-L2-
L3-Collection-v2.7z
http://fastpiratebay.eu/thepiratebay.se/torrent/9780909/Cisco-IOU-L2-
L3-Collection-v3.7z
DownloadVirtual BOX (Virtual machine emulator)
https://www.virtualbox.org/wiki/Downloads