Depending on how your infrastructure will be interconnected with the outside network, you might need to run some firewall appliances that provide access to the lab, Network Address Translation (NAT) and routing features, or even to the VPN connection.
Many vendors offer either free products or at least evaluation versions of virtual appliances:
- Cisco Cloud Services Router 1000V Series: https://www.cisco.com/c/en/us/products/routers/cloud-services-router-1000v-series/index.html
- Juniper vSRX Virtual Firewall: https://www.juniper.net/us/en/products-services/security/srx-series/vsrx/
- VyOS: https://vyos.io
- MikroTik RouterOS: https://mikrotik.com/software
You might even try to simulate complex physical networks to test some advanced networking features such as leaf-spine design or VXLANs. To do this, you will need to deploy a specific virtual machine that will act as a switch for your environment.
Cumulus Linux is one of the most frequently deployed network operating systems that can be installed within the virtual machine as well. This can be found at the following link: https://cumulusnetworks.com/products/cumulus-linux/.