Cisco Packet Tracer is a networking simulation tool developed by Cisco. It allows user to build up virtual topologies and emulate designed computer networks. This includes the emulation of Cisco Internetwork Operating System (IOS)#.
When dealing with router, especially installing modules such as serial interface (HWIC-2T) on it, switch off the router first. If the module installation has been completed, then the router is safe to turn on.
In a lab environment, it is recommended to disable Domain Name Server (DNS) lookup with the command no ip domain-lookup on the router. It is to speed up the configuration process as the router will try to search anything that it don’t know on a TFTP server. Often, the server doesn’t exist, and without the command, the router will waste times on figuring out things that it doesn’t understand, typically a wrong typed command.