The Net Sim Network Simulator for CCNA product contains CCNA-specific lab exercises that cover the skill set you will need to prepare for your CCNA exam. You can design your own network with up to 200 devices.Currently, over 40 different Cisco router models are supported, with more being added all the time. If you are planning to go for your CCNP, then you should consider the Boson Net Sim for CCNP, which includes all the CCNA-level labs plus CCNP-level labs and CCNP-level functionality.
Assuming there is an existing network, the next step is to inventory the network, identifying the circuits and all associated hardware (routers, firewalls) and software supporting the network, and summarizing all network-related expenses.
Clearly the WAN must be designed to accommodate traffic generated at each network node as well as from remote users, plus traffic across the inter-node channels.
Once the incoming and outgoing traffic statistics are compiled, the network topology is defined, and relevant operational and business-related data are identified, the new network architecture's design can begin.
GNS3 has a large user base, made up mostly of people studying for Cisco exams, and there is a lot of information freely available on the web about using GNS3 to simulate Cisco equipment.
Net Sim is a powerful and comprehensive network simulation utility that can assist you in designing an efficient network architecture.