Internet

The internet grew out of ARPANET, which carried its first messages in 1969 using the Network Control Program. On 1 January 1983 ARPANET switched to TCP/IP in a coordinated "flag day", which is why 1983 is usually given as the internet's birth year. Academic backbones such as NSFNET followed, and the commercial internet took over after NSFNET's backbone was retired in 1995. The World Wide Web, created by Tim Berners-Lee at CERN around 1989-1991, is an application running on the internet, not a synonym for it; email, DNS, VPNs and industrial telemetry use the same infrastructure.

Technically the internet is a mesh of autonomous systems (AS): networks under one administrative routing policy, each identified by an AS number. They exchange reachability information with BGP-4 (RFC 4271), and routing between them follows business policy rather than shortest paths: customers pay providers for transit, while networks of similar size often exchange traffic settlement-free through peering, frequently at internet exchange points. Unique identifiers are coordinated hierarchically: IANA allocates address blocks and AS numbers to five Regional Internet Registries (AFRINIC, APNIC, ARIN, LACNIC and RIPE NCC, which serves Europe including Denmark, the Middle East and parts of Central Asia), and the DNS root zone anchors the name space.

The architecture rests on a few design choices. IP offers only best-effort datagram delivery, with no guarantees of delivery, order or timing; reliability is added in the end hosts, following the end-to-end argument of Saltzer, Reed and Clark (1984). IP is the "narrow waist" of an hourglass: many link technologies below, many applications above. The core routers keep no per-connection state, which made the network scalable and resilient but left security out of the original design.

Several persistent weaknesses follow from that. IP does not authenticate source addresses, enabling spoofing and reflection DDoS attacks unless networks apply ingress filtering (BCP 38). BGP accepts route announcements largely on trust, so misconfigurations and hijacks can redirect traffic for whole prefixes, as when Pakistan Telecom's announcement took YouTube offline globally in 2008; Resource Public Key Infrastructure with route origin validation (RFC 6811) is the main countermeasure. For an organisation, being connected means being continuously scanned: search engines such as Shodan and Censys index exposed services, and newly exposed services are typically probed within hours. External attack surface management, redundant upstream connections and DDoS protection are therefore standard controls at the point where the internal network meets the internet.