If you are going to take a networking exam like CCNA, CCNP... it is better to practice with real routers. But when the budget is not big enough or you can only access to hardware in your school then router simulator softwares are the best things you can get for free. The followings are a number of free and popular router simulators you can download and practice with:
GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks. It is an excellent complementary tool to real labs for Cisco network engineers, administrators and people wanting to pass certifications such as CCNA, CCNP, CCIP or CCIE. This project is an open source, free program that may be used on multiple operating systems, including Windows, Linux, and MacOS X.
Screen shots:


GNS3 - GNS3 is a graphical network simulator that allows simulation of complex networks. It is an excellent complementary tool to real labs for Cisco network engineers, administrators and people wanting to pass certifications such as CCNA, CCNP, CCIP or CCIE. This project is an open source, free program that may be used on multiple operating systems, including Windows, Linux, and MacOS X.
Screen shots: