GNS3 is one of the Network Simulators, that allows you to use real router images and practice on real router CLI. Generally it is used for Cisco devices with IOS images. But you can also add Juniper JUNOS and Nokia TiMOS images, and you can use this devices on your simulated network.
GNS3 is especially an important network simulator for you during your certification studies. For Cisco CCNA, CCNP, CCIE or For Juniper JNCIA, JNCIP, JNCIE or for Nokia certifications like NRS I, NRS II and SRA, you can use GNS3 for hand on experience. With this Network Simulation Pragram, you can practice on various vendor products and increase your experience on various CLIs. You can download GNS3, here www.gns3.com
Cisco Packet Tracer is the other example of Network Simulators, in which there are many routers, switches and various network devices. You can build your topology with Packet Tracer and you can configure Cisco IOS based devices easily.
Cisco Packet Tracer is developed by Cisco Systems and it is especially very good tool for Cisco Network Certification studies. With Packet Tracer, you can practice on Cisco Routers, swithes, Access Points etc. It is especially perfect configuration practice tool for beginner network engineers.
There are many Cisco Configuration Examples with Cisco Packet Tracer on IPCisco.com beside a video CCNA Labs Course and a text based Packet Tracer Configuration Course. You can download Cisco Packet Tracer, here www.netacad.com
Cisco VIRL is the useful and powerful network simulation platform of Cisco. With Cisco VIRL, you can model your network with Cisco VIRL and you can practice on Cisco devices with this excellent Network Simulator Program. You can check Cisco VIRL, here virl-dev-innovate.cisco.com
Huawei eNSP is the useful Network Simulator platform of Huawei. With Huawei eNSP, you can practice on Huawei routers and switches. You can learn Huawei CLI, you can build various network topologies and test your network scenarious.
Huawei eNSP is especially perfect plartform for you HCIA, HCNP and HCIE Certification studies. It is easy to use network simulator platform. There is also a Huawei Configuration Course with eNSP on IPCisco. You can download Huawei eNSP, here www.huawei.com
EVE-NG (Emulated Virtual Environment) is an important platform with which you can practice on network device with various topologies. It is a strong Network Emulation Program and one of the Network Simulators for both companies or individuals. You can download EVE-NG, here www.eve-ng.net