One way I’ve learned was use existing computer equipment (old pc’s), install nics (2 ports needed), and various flavors of Linux. You in essense would turn a regular PC into a router/firewall. Now using another machine, you can use a free virtualization product of your choice. I’ve used Ubuntu on a laptop with VMware Server. On the laptop, I have various systems configured but due to CPU & Memory restraints, I would only turn up 3 VMs at a time.
I would started to volunteer or do co-op at companies like Cisco or IBM on they community projects.