Cisco CCNA 200-125

GNS3 https://gns3.com/

https://github.com/GNS3/gns3-gui/releases/download/v2.0.3/GNS3-2.0.3-all-in-one.exe

install 2.0.3 !

you can download IOS image here https://srijit.com/working-cisco-ios-gns3/ (download 7200 and 3745 )

in GNS3, go to Edit/Preferences/Dynamips/IOS routers/

also, GNS3 is a spiteful beast, I managed to make it work only by copying the "bin" files to C:\Users\Pierre-Luigi\GNS3\images\IOS and by running gns3server.exe in a cmd dos prompt. What a piece of crap!
Otherwise it will tell you

"Could not create IOS router: Error while setting up node: S:/pierre/downloads/c7200-advipservicesk9-mz.152-4.S5.bin is not allowed on this remote server. Please use only a filename in C:\Users\Pierre-Luigi\GNS3\images\IOS.
error while deleting : Node ID 4f03d98e-bcca-4623-a2dc-9d5095eefb64 doesn't exist
Could not create IOS router: Node ID 4f03d98e-bcca-4623-a2dc-9d5095eefb64 doesn't exist
"

GNS3 allows you to construct and test networks in a risk-free virtual environment without the need for network hardware


CISCO packet-tracer https://www.itechtics.com/packet-tracer-download/ "a powerful network simulation software from Cisco Network Academy which can simulate/create a network without having a physical network" (with Netacademy you can take free course on Packet Tracer)
and download Packet Tracer here https://www.netacad.com/portal/resources/packet-tracer


Switch
Router
Firewall
Local Area Network
Wide Area Network

OSI Model (ISO standard): Application, Presentation, Session, Transport (Port), Network (IP -> ROUTER)), Datalink (MAC -> SWITCH), Physical
please do not throw sausage pizza away


TCP-IP stack: Application, Transport. Internet, Network access

PDU (protocol data unit): data, segment, packet, frame


Network Calculator http://jodies.de/ipcalc?host=200.15.10.1&mask1=27&mask2=

http://subnettingquestions.com/

http://subnetting.org


https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html EIGRP