Nonroutable / Non-Routable / non-routable otherwise known as Private Network Address Space.
Non-routable address space allows for nodes to communicate with each other on same network and will not conflict to other network addressing on the Internet. IP packets originating from or addressed to a private IP address cannot be routed through the public Internet.
Both the IPv4 and the IPv6 specifications define private IP address ranges. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Private network addresses are not allocated to any specific organization. Private IP address spaces were originally defined to assist in delaying IPv4 address exhaustion.