nonroutable

Nonroutable is a term used in computer networking to describe addresses, prefixes, or resources that are not intended to be reachable via public routing on the Internet. Nonroutable designations are used to isolate internal networks, conserve globally unique address space, and control how traffic is forwarded between networks. A resource may be considered nonroutable if routers on the wider Internet should not forward packets toward it.

In IPv4, the private address ranges defined by RFC 1918 are widely used as nonroutable addresses within enterprises and home networks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These addresses are not expected to be routed on the public Internet and are typically reached through internal routers or NAT when communicating with external networks.

Other IPv4 blocks are nonroutable because they are reserved for special purposes. This includes 127.0.0.0/8 (loopback), and 169.254.0.0/16 (link-local addressing used when a device cannot obtain an address from a DHCP server).

Additional nonroutable or non-public blocks include 100.64.0.0/10 (shared address space for carrier-grade NAT) and 198.18.0.0/15 (TEST-NET for benchmarking). These addresses are not meant to be globally routed and are intended for private operations or testing within controlled networks.

In IPv6, nonroutable concepts persist through link-local and private addressing. Link-local addresses, fe80::/10, are used only on a single link and are not routable beyond it. Unique local addresses, fc00::/7, are intended for private networks and are not globally routable, though they may be routed within an organization.

Understanding nonroutable addressing helps manage address space, security, and connectivity. Separation of private and public routing domains remains a central principle of Internet routing, with NAT and firewalls often mediating traffic between nonroutable networks and the global Internet.