Flat network: It is often used in private clouds since it is very easy to set up. The downside is that any virtual machine will see any other virtual machines in our cloud. I strongly discourage people from using this network design because it's unsafe, and in the long run, it will have problems, as we have seen earlier.

VLAN: It is sometimes used in bigger private clouds and sometimes even in small public clouds. The advantage is that many times you already have a VLAN-based installation in your company. The major disadvantages are the need to trunk ports for each physical host and the possible problems in propagation. I discourage this approach, since in my opinion, the advantages are very limited while the disadvantages are pretty strong.

VXLAN: It should be used...