the default-free zone

Most networks, when they don't know exactly where to send a packet, fall back on a default route — "if in doubt, send it upstream to my provider and let them figure it out." That works because someone above them knows more. But who do the networks at the very top fall back on? Nobody. The default-free zone is the part of the Internet made up of routers that have NO default route, because they hold complete routing information: a route to every reachable prefix on the public Internet.

These are the routers in the core — the tier-1 backbones and others who carry full BGP tables. To forward correctly without any "if in doubt" fallback, each must know an explicit BGP route for every destination prefix in the world. The collection of all those prefixes is the global routing table, and its size is one of the most-watched numbers in networking. It has grown relentlessly: it crossed 512,000 IPv4 routes in 2014 (a milestone that crashed older routers whose memory tables overflowed at exactly 512K), and the IPv4 table is now well past a million routes, with the IPv6 table climbing too.

The default-free zone matters because it sets the hardware demands of the Internet's core. A router here must store every global prefix and compute a best path for each — so its memory (especially fast forwarding memory) and its ability to absorb a constant churn of updates set real limits. This is also why route aggregation (CIDR) is so important: combining many small prefixes into one larger advertisement keeps the global table from exploding even faster than it already does.