routable

Routable refers to whether a network address or packet can be forwarded by routers across network boundaries to reach a destination on another network. A routable address is one that can be reached through the global or inter-domain routing system, with routes advertised in routing tables. Non-routable addresses, by contrast, are intended for use within a single local network or link and are not expected to be reachable from the wider Internet.

In IPv4, non-routable addresses include the private address spaces defined in RFC 1918: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These are commonly used inside private networks and are not globally routable. A gateway or network address translation (NAT) is typically used to access public networks. Link-local addresses (169.254.0.0/16) are also non-routable beyond the local link. Public IPv4 addresses are routable on the Internet and are learned through global routing protocols such as BGP.

In IPv6, global unicast addresses are routable on the Internet, while unique local addresses (ULA, fc00::/7) are intended for private use and are not globally routable. Link-local IPv6 addresses (fe80::/10) are used for local link communication and are not routed.

Routability depends on actual routing information in devices and networks. Address allocation policies, route aggregation, and interdomain routing protocols determine which addresses are reachable across administrative boundaries. NAT and VPNs can extend reachability for non-routable addresses in controlled ways.

See also: NAT, RFC 1918, IPv4 address space, IPv6 addressing architecture, routing table, BGP.