The IP envelope

At the moment you click a link, your browser packages your request into an IP packet — an envelope with your address and the server’s address. Here is the radical part: no single router between you and that server knows the complete path. Each router just looks at the destination address, consults its own local map, and forwards the packet to the next hop.

What IP does in one sentence

IP is the addressed envelope every chunk of Internet traffic travels inside. Source address, destination address, payload, and a small header — that’s IP.

Every protocol you use — TCP, UDP, ICMP, every video chunk, every webpage — rides inside IP packets. The packet is the unit of routing for the entire Internet.

The postal metaphor

Imagine the postal system. Each letter has a from-address and a to-address. Local post offices don’t know where Tokyo is, but each one knows “anything for Asia goes via this truck.” Letters hop from local office to regional to national until they reach the destination’s local office. No single office holds a map of the whole world — only the next hop.

IP works the same way. Routers hold partial maps and make local decisions. At each hop the router reads the destination address, checks its routing table, and forwards the packet to whichever neighbour is the best next step. After 10–15 such hops the packet arrives.

One scenario end to end

Your browser already finished the physical link (it knows how to put bits on a wire). It now builds an IP packet for every segment it wants to send. Each packet travels independently. The layers above — TCP, TLS, HTTP — all hand bytes to IP, which adds the envelope.

IPv4 vs IPv6

IPv4 uses 32-bit addresses written as four decimal octets (e.g. 93.184.216.34). IPv6 uses 128-bit addresses written as eight 16-bit hex groups (e.g. 2001:db8::1). The practical differences:

• Address space. IPv4 has ~4.3 billion addresses — exhausted. IPv6 has 340 undecillion — effectively unlimited.
• Header. IPv4 has a variable header with a checksum computed at every hop. IPv6 has a fixed 40-byte header with no checksum (delegated to L2 and L4) and no router-level fragmentation.
• Adoption. Most new deployments run both (dual-stack). IPv6 reached ~45–50% of global Internet traffic by 2026.