Non-routable Internet addresses

Posted in Networking on

This article describes IPv4 address ranges that are not globally routable on the public Internet. Each range is reserved by IETF standards (RFCs) for specific technical, operational, or documentation purposes and is commonly encountered in real-world networking scenarios such as NAT, VPNs, testing, and local communications.

0.0.0.0/8

This range is defined in RFC 1122, RFC 3330, and RFC 1700.

The address 0.0.0.0 represents an unspecified IPv4 address, and its meaning depends on context.

Common usage scenarios:

  • Default route. In routing tables, 0.0.0.0/0 represents the default route, matching any destination not covered by a more specific route.
  • Service binding. When a service binds to 0.0.0.0, it listens on all available network interfaces, which is common for servers, containers, and VPN gateways.
  • Bootstrapping and discovery. During early network initialization (for example, DHCP discovery), a device may use 0.0.0.0 as a temporary source address before a valid IP is assigned.
  • Offline or unconfigured state. Some operating systems display 0.0.0.0 when no network configuration is present.

10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 – Private IPv4 address space (RFC 1918)

These address ranges are reserved for private internal networks and are not routed on the public Internet, as defined in RFC 1918.

Typical real-world usage:

  • Corporate networks. Large organizations often use 10.0.0.0/8 for internal segmentation across offices, data centers, and cloud environments.
  • Home and small office networks. Consumer routers commonly use 192.168.0.0/16, with 192.168.0.1 or 192.168.1.1 acting as the default gateway.
  • VPN environments. Private ranges are frequently transported through site-to-site or client VPN tunnels. Care must be taken to avoid address space overlap between connected networks.

To communicate with external networks, Network Address Translation (NAT) is used, allowing private addresses to be mapped to one or more public IP addresses.

100.64.0.0/10 – Carrier-Grade NAT (CGNAT)

Defined in RFC 6598, this range is reserved for shared address space between service providers and subscribers.

Practical scenarios:

  • ISPs assign addresses from this range to customers when public IPv4 addresses are scarce.
  • Traffic is translated again at the provider edge, creating a double NAT scenario.
  • Frequently encountered when hosting services behind residential or mobile connections, where inbound connections are restricted or impossible.

    • 127.0.0.0/8 – Loopback

    Allocated by RFC 1122, this range is reserved for loopback communication within a host.

    Key characteristics:

    • Traffic never leaves the local machine.
    • Used for local service testing, development environments, and health checks.
    • Commonly used in container platforms and service meshes to communicate between local components without external network dependency.

    169.254.0.0/16 – Link-local (APIPA)

    Defined in RFC 3927, this range is used for automatic private IP addressing when no DHCP server is available.

    Typical scenarios:

    • A device automatically selects an address from this range after DHCP failure.
    • Used for temporary connectivity, diagnostics, or direct device-to-device communication.
    • Commonly seen during network outages or initial infrastructure provisioning.

    192.0.0.0/24 – IETF protocol assignments

    Reserved by RFC 6890, this block is used for special-purpose protocol functions and is not intended for general host assignment.

    192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 – Documentation and example networks (TEST-NET)

    According to RFC 5737, these ranges are reserved exclusively for documentation and examples.

    They are commonly used in:

    • Technical documentation
    • Configuration samples
    • Training materials and presentations

    Example:

    
ping 192.0.2.5
curl http://203.0.113.7

    192.88.99.0/24 – 6to4 relay anycast (deprecated)

    This subnet was reserved for 6to4 IPv6 transition relays as defined in RFC 6219.

    The 6to4 mechanism is now deprecated, and many networks actively filter this range.

    It remains of historical and educational interest rather than practical use.

    198.18.0.0/15 – Network benchmarking

    Allocated by RFC 2544 and clarified in RFC 6815, this range is used for performance and stress testing.

    Common use cases:

    • Benchmarking routers, firewalls, and load balancers
    • Simulating large-scale traffic without risking conflicts with real networks

    224.0.0.0/4 – Multicast

    Historically known as Class D, this range is reserved for IP multicast as described in RFC 5771.

    Used by protocols such as:

    • OSPF
    • mDNS
    • SSDP
    • Multicast routing protocols

    Multicast addresses are not globally routed in the same way as unicast traffic and serve controlled, protocol-specific purposes.

    240.0.0.0/4 – Reserved (Class E)

    Defined in RFC 1112, this range is reserved for future use and is not currently deployed in public networking.

    Summary

    Non-routable IPv4 address ranges exist to support private networking, testing, protocol design, and transitional technologies. Understanding where and why these addresses appear is essential when designing networks, troubleshooting connectivity issues, configuring NAT or VPNs, and interpreting packet captures.