How the internet works: from a tap on your phone to a web page

Your phone connects to a local network: a mobile tower or a Wi-Fi router. That network belongs to an internet service provider (ISP), which in turn connects to other, larger networks. Where networks meet and exchange traffic, usually in large data centres, the points are called internet exchanges. India has several, run under the National Internet Exchange of India and by private operators, and keeping traffic between Indian networks inside the country makes it faster and cheaper than sending it abroad and back.

Every device on the internet has an IP address, a number that says where to send data. The older version, IPv4, has four numbers such as 192.0.2.1 and allows about 4.3 billion addresses, which have run out. The newer version, IPv6, uses much longer numbers and allows more addresses than there are grains of sand on Earth.

People use names, not numbers. The Domain Name System (DNS) is the internet's phone book: when you type a name, your device asks a DNS resolver, often run by your ISP, for the matching IP address. Answers are remembered for a while, so most lookups finish in a few milliseconds.

Sending a whole photo or web page as one block would be fragile and slow, so the data is cut into packets of a thousand bytes or so, each labelled with where it is from and where it is going. Routers along the way read the label and pass each packet towards its destination, and different packets of the same message may take different routes.

At the other end a protocol called TCP puts the packets back in order and asks for any that were lost to be sent again. Some applications, such as video calls and online games, use a faster protocol called UDP, which does not wait for lost packets, because a late packet is no use in a live call. Newer systems such as QUIC combine speed and reliability.

When you open a website, roughly this happens:

• Your browser asks DNS for the IP address of the site.
• It opens a connection to that address and agrees an encryption key with the server in a handshake. The server proves its identity with a certificate issued by an authority your browser trusts. This is the S in HTTPS.
• The browser sends an HTTP request for the page, and the server replies with the page's HTML.
• The HTML points to more files: styles, scripts, fonts and images. The browser requests those as well, often from several servers.
• The browser assembles everything and draws the page.

The great majority of traffic between continents travels through fibre-optic cables, glass threads that carry pulses of light, many of them laid along the sea floor. Mumbai and Chennai are among the main places where these cables come ashore in India. Within a country, fibre links cities and towns, and the last stretch to your phone is radio, from a 4G or 5G tower. Satellites serve ships, aircraft and places that cables do not reach.

A content delivery network (CDN) keeps copies of popular pages and videos on servers close to users, so that a page about a match is served from a nearby city and not from another continent.

Light travels through glass at about 200,000 kilometres a second, so every 1,000 kilometres costs about 5 milliseconds one way. That puts a floor under delays: a server on another continent can never be as quick as one in the next city. Beyond distance, delay comes from crowded networks, weak Wi-Fi, a busy server, or a long chain of requests that must be made one after another. When a page is slow, the fault can be at any of these steps, which is why fixing it begins with finding out which one.

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