I use GNS3 (Graphic Network Simulator 3) and Oracle Virtual Box for practicing for my CCNP/CCIE/ and CISSP cert exams. You *WILL* need legitimate IOS images for the Cisco Routers (2621XM, 3640, and 7200), but with Oracle VBox (it's free), you can run a plethora of OSes to simulate servers and desktops. I've been able to run OSX Mountain Lion, OSX Maverick, Ubuntu, Windows2008R2, OpenSolaris 12, XP Pro, Haiku, Plan 9, Linux Mint Cinnamon, Cloanto Amiga Forever, Novell Netware 5 and Win NT4SP6a with no issues at all.

I'm willing to point out where you can obtain the software you need, but if it violates the site TOS, please let have an admin let me know, because the last thing I want to do is want to get VOGONS in trouble.

Just in case it get nuked, PM me, and I'll do what I can to help. 90% of the software is freeware, but the IOS versions are dodgy, but can be found, if you know where to look. IF you actually own cisco routers, it;s perfectly legal to extract them from a router that you own. Cisco Systems will even walk you through the procedure.