Why Global Carriers Drop Prefixes Smaller than /24 (IPv4) & /48 (IPv6)
The global internet routing table (the Default-Free Zone, or DFZ) currently contains over 950,000 IPv4 routes. To prevent route table explosion and core router memory exhaustion, global transit operators enforce strict filtering policies dictated by RFC 7454 (BGP Operations and Security).
- The /24 IPv4 Minimum: An organisation cannot announce a /25 (128 addresses) or smaller block to transit providers. If you advertise a /25, Tier-1 upstreams (such as Lumen, Arelion, NTT, and Colt) will discard the advertisement at their ingress filters.
- The /48 IPv6 Minimum: While IPv6 provides massive address space, the minimum routable block in the DFZ is /48 (representing 65,536 /64 subnets). Announcements of /56 or /64 will not propagate globally.
- RPKI ROA Validation: Modern BGP peering requires publishing Route Origin Authorisations (ROAs) within regional registries (RIPE NCC in the UK/Europe). Routers running RPKI Route Origin Validation (ROV) instantly mark unauthorized origins or invalid prefix lengths as Invalid and drop them.
Traffic Steering: AS-Path Prepending vs Local-Preference
In multi-homed enterprise WANs with dual carrier leased lines, steering inbound traffic requires influencing external autonomous systems:
- Local-Preference (Outbound Steering): A metric internal to your AS. Higher values win (e.g. 200 vs default 100). Dictates which circuit your users and servers use when sending data to the internet.
- AS-Path Prepending (Inbound Steering): Artificially repeats your own ASN in the AS-Path attribute to make a secondary circuit appear longer and less desirable to remote internet networks.
- RFC 7999 RTBH (DDoS Blackholing): When an individual IP address faces a volumetric DDoS attack, tagging that specific host (/32) with BGP community
65535:666instructs your carrier to drop the traffic at their transit edge, preventing circuit saturation. Explore our guide to Enterprise DDoS Mitigation & BGP Anycast.