Configuring Proxmox to use NAT Networking
By default, Proxmox configures your primary NIC (Network Interface Card) to work in bridged mode. For normal installations, this is the best configuration - VMs and CTs that you create automatically get assigned a DHCP address from your router so can easily access your local network.
In some cases however, bridged networking may be unsuitable (if you are connected to a local network with a low number of free IP addresses) or impossible to use. Many Dedicated Server providers say that you need to purchase a secondary external IP address in order to run Proxmox in bridged mode. With NAT mode you can get away with only using a single external IP address.