the Internet Protocol
/ I-P /
The Internet Protocol, IP, is the single agreement that lets a billion different machines on countless different networks all talk to one another. It is the narrow waist of the Internet: everything from email to video runs over IP, and IP runs over everything from fibre to Wi-Fi. Picture it as the universal envelope format that every post office in the world has agreed to recognize, so a letter can pass from one country's system to the next without being repackaged.
IP defines two things. First, an addressing scheme: every interface gets an IP address (32 bits in IPv4, 128 bits in IPv6) so any host can be named globally. Second, the datagram format and the forwarding rules: how to build an IP datagram (a header plus payload), and how each router should read the destination address and pass the datagram toward it. IP offers a connectionless, best-effort service — it tries its best to deliver each datagram but promises nothing about delivery, order, or integrity of the data.
Because IP promises so little, it can run almost anywhere and stay simple and fast — that minimalism is its genius, not a flaw. Anything stronger is layered on top: TCP adds reliability, TLS adds encryption, ICMP carries error reports. The two versions in use are IPv4 (still dominant, but its 32-bit addresses ran out) and IPv6 (128-bit, designed to replace it). When people say a service is on the Internet, they almost always mean it speaks IP.
Whether your packet rides over fibre, a 5G radio link, or office Ethernet, it carries the same IP header. Each link layer treats the IP datagram as opaque cargo; only the source, the routers, and the destination read the IP address inside.
One envelope format spanning every kind of link — the Internet's common language.
IP by itself is not reliable and not secure: it neither retransmits lost data nor encrypts anything. Reliability comes from TCP and security from TLS or IPsec, layered above IP.