When is using private IPv4 addressing appropriate for a new subnet?

Private IPv4 addresses (RFC 1918) are designed for internal networks and are not routable on the public internet. Therefore, they are appropriate when an organization has limited public IPv4 address space and the traffic within the new subnet is intended to remain entirely within the organization's local network. This conserves public IP addresses. Using private addressing is not about limiting broadcasts; that's typically handled by subnetting and VLANs. If an ISP requires a subnet to be advertised to the internet, public IP addresses are necessary. While traffic traversing a site-to-site VPN can use private addresses on both ends, the decision to use private addressing for a new subnet is primarily driven by the need for internal-only communication and conservation of public IP space, not the VPN itself.