Packet tracer is definitely not OSS nor free as in speech. But it is probably your best bet for something to get started with. You can go on netacad and get it for free together with the self-paced introduction to packet tracer course. I'm not trying to shrill for Cisco, they're a soulless company that exploits their position to extort insane prices. But PT is kinda nice, IF! all you want is to build simple networks and you don't care about being vendor agnostic or only running FOSS.

PS: and cisco? 2900€ for a 24port l3 switch? GTFO, a mikrotik crs326 can do the same and is 200€ and it doesn't require a DNA-center to enable licensing, why? BECAUSE IT DOESN'T REQUIRE ADDITIONAL LICENSES!!1!one!

In the same boat as the other poster, its been like 10 years.

I used GNS3 and Cisco VIRL way back in the day.

Depending on your use case, you can pretty far with just docker and some Linux packages. I've done GRE, BGP, OSPF, ISIS, Open vSwitch. That's Linux networking though. If you're trying to prep for a specific vendor's cert, it might not meet your needs.

Does look like someone had success running virtual devices in docker that might be of interest: https://github.com/vrnetlab/vrnetlab

Sorry, packet tracer is the only one I have experience with, and that was 10 years ago during a sysadmin course.

I only used GNS3 and Cisco Packet Tracer from the list when self-teaching CCNA. GNS3 installation shouldn't be that hard. On Windows just use the installation and fire up a VM in VBOX / HyperV (recommended) and it should automatically detect it. On Linux I have installed GNS3 using AUR package in ArchLinux, haven't tried on Ubuntu, but it should be straight forward. For the VM, you will need hypervisor setup (virt-manager, libvirtd) and KVM enabled. Most of the distrobutions should have its own kvm virtualization guide. You can also skip GNS3 installation on the host and just go to IP of the VM,so you can use beta Web UI where you can do most of the things you can do in GNS3 app. Good luck!