Explainer cheatsheetFrom your phone to the ocean floorSimple, analogy, technical
Internet
Unpacked
You tap a link and a page appears in under a second. In between, your request is cut into packets, sent by radio to a tower, turned into light, carried across the sea floor, passed between dozens of routers and rebuilt in order. Every step below comes three ways: in simple words, as an everyday picture, and in full technical detail.
The journey of one requestTap a box to jump to its module
What the internet is
Not one network owned by anyone, but tens of thousands of separate networks that agreed to pass each other's traffic.
In simple words
The internet is many separate networks joined together. Your mobile company runs one, your office runs one, Google runs one. They connect at shared meeting points called exchanges and agree to carry each other's data.
Think of it like
It is like the world's road system. No single company owns every road. City streets, state highways and national expressways are built by different owners, but they join at interchanges, so a truck can drive from your street to any other street in the country.
Swipe sideways to see the whole diagram
- 1Each box is an autonomous system: one network with its own owner and rules.
- 2The amber disc is an Internet Exchange Point, a building where networks plug into a shared switch.
- 3The dashed line is transit: a small network pays a big one to reach everyone else.
Technical deep diveTap to fold
- Autonomous System (AS)
- A network under one administration with its own routing policy, identified by an AS number such as AS13335. Tens of thousands exist.
- Peering
- Two networks exchange traffic directly, usually for free, because both benefit. It often happens at an IXP such as NIXI in India or DE-CIX in Frankfurt.
- Transit
- A paid service where a larger network carries your traffic to the rest of the internet.
- Tier 1 network
- A backbone that reaches every other network through peering alone, without paying anyone for transit.
- Internet vs web
- The internet is the network that moves packets. The web is one service on top of it, alongside email, video calls and games.
Packets: data in small envelopes
Nothing travels the internet whole. Every photo, message and video is cut into small numbered pieces called packets.
In simple words
Your data is cut into small pieces called packets. Each piece gets a header with where it came from, where it is going and its number in the line. The pieces travel on their own and are joined back together at the end.
Think of it like
Imagine posting a 300 page book through the mail, one page per envelope. Each envelope carries the address, the return address and a page number. They may take different trucks and arrive out of order, but the page numbers let the reader rebuild the book.
Swipe sideways to see the whole diagram
- 1The file is split into pieces that fit the MTU, usually 1,500 bytes.
- 2Each piece gets headers, like labels on an envelope.
- 3Layers wrap each other: this is encapsulation. Each device reads only the layer it needs.
Technical deep diveTap to fold
- Packet switching
- Links are shared by everyone, packet by packet, instead of reserving a whole circuit per call as old phone networks did. That is why the internet is cheap and resilient.
- MTU
- Maximum Transmission Unit, the largest packet a link carries. 1,500 bytes on Ethernet. IPv4 and TCP headers use 40 of them, leaving a 1,460 byte MSS for data.
- IP header
- Source and destination address, TTL (a hop counter that stops packets looping forever), protocol number and length.
- TCP header
- Source and destination port (which app), sequence number (where this data sits in the stream), acknowledgement number and flags such as SYN and ACK.
- Layers
- Link (Wi-Fi, Ethernet), Internet (IP), Transport (TCP, UDP), Application (HTTP, DNS). Each layer only talks to the same layer on the other side.
Addresses and DNS
Every device has a numeric IP address. DNS is the lookup service that turns names people remember into those numbers.
In simple words
Computers find each other by number, an IP address, not by name. When you type a website name, your phone first asks a lookup service, DNS, for that site's number, then sends packets to the number.
Think of it like
DNS is your phone's contact list. You tap "Mom", the phone looks up the number and dials it. You never need to remember the digits, and if Mom changes her number, only the contact entry changes.
Swipe sideways to see the whole diagram
- 1Your phone asks the recursive resolver for letterpad.io.
- 2The resolver asks a root server, which points to the .io servers.
- 3The TLD server for .io points to the site's own name servers.
- 4The authoritative server replies with the IP address.
- 5The resolver answers your phone and caches the answer for next time.
Technical deep diveTap to fold
- IPv4
- A 32 bit address like 104.21.3.7, about 4.3 billion in total, which ran out years ago.
- IPv6
- A 128 bit address like 2606:4700::6815:307, enough for every device many times over. Most Indian mobile networks hand out IPv6 today.
- NAT
- Network Address Translation lets many devices share one public IPv4 address. Your router does it at home; mobile carriers do it at scale as CGNAT.
- DHCP
- How a device gets its IP address, gateway and DNS server automatically when it joins a network.
- DNS record types
A(IPv4),AAAA(IPv6),CNAME(alias),MX(mail),TXT(verification).- TTL in DNS
- How many seconds an answer may be cached. Short TTLs let you move a site quickly; long ones make lookups faster.
Try it: dig +trace letterpad.io prints every step of the chain above.
The cell tower
On mobile data, the first hop is radio. The tower turns your phone's radio signal into packets on a fibre line to the operator's core network.
In simple words
Your phone talks to the nearest cell tower using radio waves. The tower is not the internet itself, it is a doorway. Behind it a backhaul cable carries your data to the mobile company's central computers, the mobile core, which check your SIM, give you an address and send your traffic out to the internet.
Think of it like
The tower is a post box at the end of your street. You drop a letter in (radio), a van collects it (backhaul fibre), and it goes to the main sorting office (mobile core), which checks the stamp (your SIM) and sends it on its way.
Swipe sideways to see the whole diagram
- 1Your phone sends radio signals on a licensed frequency band.
- 2The tower (base station) converts radio into data and sends it over backhaul, usually fibre, sometimes microwave links.
- 3The mobile core authenticates your SIM, assigns your IP and connects you to the internet.
- 4Handover moves you between towers as you travel.
Technical deep diveTap to fold
- RAN
- Radio Access Network: the towers and antennas. 4G calls a base station an eNodeB, 5G a gNodeB.
- Cells and spectrum
- Each tower covers areas called cells, using frequency bands such as 700 MHz (wide reach) or 3.5 GHz (fast, short range). Neighbouring cells use different frequencies to avoid interfering.
- OFDM
- The radio scheme behind 4G, 5G and Wi-Fi: data is split across thousands of narrow sub-carriers at once, so interference on one only hurts a little.
- Mobile core
- In 5G: the AMF handles login and mobility, the SMF sets up your data session, and the UPF forwards your actual packets to the internet. 4G used the MME, SGW and PGW.
- SIM authentication
- The SIM holds a secret key also stored at the operator. The network sends a challenge and checks your SIM's answer, so the key itself never travels.
- Latency
- The radio hop adds roughly 10 to 50 ms on 4G and 5G. Wi-Fi and fibre at home are usually a few ms.
Wi-Fi, router and modem
At home, two boxes do two different jobs. The router manages your devices. The modem speaks the language of the line coming into your house.
In simple words
Your phone reaches the router by Wi-Fi radio. The router gives every device its own local address and sends their traffic out through one connection. The modem then turns that data into the kind of signal the outside line carries: light for fibre, electrical waves for cable or phone lines.
Think of it like
The router is the receptionist of a building, who knows which desk every visitor wants and routes calls to the right person. The modem is a translator at the front door, turning everything said inside into a language the outside world understands, and back.
Swipe sideways to see the whole diagram
- 1Devices join the Wi-Fi network and get local addresses from the router's DHCP.
- 2The router uses NAT so all devices share one public address.
- 3The modem (or ONT on fibre) converts data to the line's signal and back.
Technical deep diveTap to fold
- Modem
- MOdulator DEModulator: it encodes bits onto a carrier wave and decodes them back. Cable modems use DOCSIS, phone lines use VDSL2.
- ONT
- Optical Network Terminal: the fibre version of a modem. It turns light from a GPON or XGS-PON fibre into Ethernet. One fibre from the exchange is split to up to 64 homes.
- QAM
- Quadrature Amplitude Modulation sends several bits per symbol by varying both the strength and the timing of a wave. 4096-QAM packs 12 bits per symbol.
- Wi-Fi bands
- 2.4 GHz reaches further through walls; 5 and 6 GHz are faster but shorter range. Wi-Fi 6, 6E and 7 add more channels and better sharing.
- MAC address and ARP
- Inside your home, devices are found by hardware MAC address. ARP maps an IP address to a MAC address on the local network.
- Default gateway
- The router's local address, often 192.168.1.1. Any packet not meant for a local device goes there first.
Fibre: data as light
Long distance internet travels as flashes of infrared light inside glass thinner than a hair.
In simple words
A laser blinks on and off billions of times a second. On means 1, off means 0. The light travels inside a thin glass thread, an optical fibre, and cannot escape because it keeps bouncing off the inside walls. Many colours of light can share one thread, each carrying its own stream of data.
Think of it like
Picture shining a torch into a long pipe lined with mirrors, and sending Morse code by switching it on and off. Now give a hundred friends torches of different colours, all shining into the same pipe. At the far end a prism splits the colours apart again.
Swipe sideways to see the whole diagram
- 1Light stays in the core because of total internal reflection at the cladding boundary.
- 2The cross-section: a glass core, glass cladding with a slightly lower refractive index, and a protective coating.
- 3DWDM combines many wavelengths into one fibre with a multiplexer.
Technical deep diveTap to fold
- Single-mode fibre
- A 9 µm core carries one path of light, so pulses do not smear over long distances. Used for all long-haul and subsea links.
- Wavelength
- Infrared around 1,550 nm, where glass absorbs the least light.
- DWDM
- Dense Wavelength Division Multiplexing: dozens to over a hundred wavelengths per fibre, each carrying 100 to 800 Gbit/s with modern coherent optics.
- Optical amplifier
- Light fades over distance, so every 50 to 100 km an EDFA (erbium-doped fibre amplifier) boosts all wavelengths at once without turning them back into electricity.
- Speed of light in glass
- About 200,000 km per second, two thirds of the speed in vacuum. Every 1,000 km adds roughly 5 ms one way, which is why distance sets a floor on latency.
Cables under the sea
Almost all traffic between continents crosses the ocean floor in cables about as thick as a garden hose. Satellites carry only a tiny share.
In simple words
When you open a website hosted in Europe or America, your packets leave the country through a subsea cable lying on the sea floor. The cable comes ashore at a building called a landing station, in cities such as Mumbai and Chennai, and joins the land network there.
Think of it like
Subsea cables are the international flight routes of data. Landing stations are the airports where they touch land, repeaters are refuelling stops along the way, and repair ships are the ground crew that fix a broken route.
Swipe sideways to see the whole diagram
- 1The cable comes ashore at a cable landing station, which feeds power and connects it to land networks.
- 2Amber squares are repeaters, optical amplifiers that boost the light.
- 3Most damage comes from ship anchors and fishing near the coast, where cables are buried and armoured.
Swipe sideways to see the whole diagram
Technical deep diveTap to fold
- Scale
- Roughly 600 cable systems are in service or being built, adding up to well over a million kilometres. A new system can carry hundreds of terabits per second.
- Fibre pairs
- A cable holds a small number of fibre pairs, often 8 to 24. Each pair runs DWDM, so total capacity is pairs × wavelengths × rate per wavelength.
- Power feed
- Repeaters need electricity on the seabed, so landing stations push high voltage DC, up to around 15 kV, down the copper layer.
- Armouring
- Deep-sea cable is about 17 to 20 mm thick. Near shore it gets extra steel armour and is buried in a trench, because that is where anchors and trawlers hit it.
- Repair
- A ship finds the fault from the landing stations by sending light and timing reflections, lifts both ends with a grapnel, splices in new cable and lowers it back. It can take days to weeks.
- Redundancy
- Countries use many cables on different routes. When one is cut, BGP moves traffic onto the others within seconds, usually with only higher latency.
Routing: finding the way
No router knows the whole journey. Each one looks at the destination, picks the best next hop, and forgets the packet.
In simple words
A router is a junction. It reads the destination address on each packet and sends it one step closer, along the best road it currently knows. Packets from the same file can take different roads if traffic changes, and that is fine.
Think of it like
It is like asking for directions in a new city. Nobody gives you the full route; each person you ask points you to the next street. If a road is blocked, the next person simply points another way.
Swipe sideways to see the whole diagram
- 1Each router checks its routing table and forwards to the next hop.
- 2Packets of one file may take different paths, so they can arrive out of order.
- 3If a link fails or fills up, routers reroute around it.
Technical deep diveTap to fold
- Routing table
- A list of address ranges (prefixes such as 104.16.0.0/13) and the next hop for each. The most specific match wins: longest prefix match.
- BGP
- Border Gateway Protocol: how separate networks (autonomous systems) tell each other which addresses they can reach. The global table holds around a million IPv4 routes.
- OSPF and IS-IS
- Routing protocols used inside one network to find the shortest path between its own routers.
- TTL
- Every router lowers the packet's TTL by one. At zero the packet is dropped and an ICMP message is sent back, which is exactly how
traceroutemaps a path. - Anycast
- The same IP address announced from many places. BGP sends you to the nearest one, which is how DNS roots and CDNs stay close to everyone.
Try it: traceroute -n google.com (or tracert on Windows) lists every router your packets pass.
Reassembly: back in order
Packets arrive late, early or not at all. TCP numbers every byte, puts them back in order, and asks again for anything missing.
In simple words
Your phone keeps arriving packets in a waiting area, the buffer, sorted by their sequence number. It tells the sender what it has received. If a number is missing, the sender sends that piece again: a retransmission. Only when the line has no gaps does the app get the data.
Think of it like
It is a jigsaw sent in many parcels. You lay out the pieces by number as they arrive, and when piece 4 never shows up, you call the sender: "I have everything up to 3, please send 4 again."
Swipe sideways to see the whole diagram
- 1Sequence numbers tell the receiver where each piece belongs.
- 2The receive buffer holds pieces until gaps are filled.
- 3An ACK tells the sender what arrived; a missing piece is retransmitted.
Technical deep diveTap to fold
- Three-way handshake
- Before data flows:
SYN(client),SYN-ACK(server),ACK(client). Both sides agree on starting sequence numbers. Costs one round trip. - Sequence and ACK numbers
- TCP numbers bytes, not packets. An ACK of 4,381 means "I have every byte before 4,381". SACK can also report blocks received after a gap.
- Retransmission
- Triggered by three duplicate ACKs (fast retransmit) or by a timeout (RTO) if nothing comes back.
- Flow control
- The receiver advertises a window: how much more it can buffer. The sender never sends beyond it.
- Congestion control
- The sender starts slowly and speeds up until loss or delay signals a full link, then backs off. Linux uses CUBIC by default; Google's BBR models bandwidth and delay instead.
- UDP and QUIC
- UDP skips all of this for speed (voice, games, DNS). QUIC, used by HTTP/3, rebuilds reliable delivery on top of UDP with a faster handshake and no blocking between streams.
HTTPS: sealing the envelope
Packets pass through many networks you do not control. TLS encrypts them so only your browser and the real server can read them.
In simple words
Before sending anything private, your browser and the server agree on a secret encryption key that nobody watching can work out. The server also proves its identity with a certificate. After that, every packet is scrambled for anyone except the two of you.
Think of it like
It is like sending documents in a locked box. You and the receiver each mix half of a lock combination in public, yet only you two end up with the full combination. A trusted notary's stamp on their ID proves you are locking it for the right person.
Swipe sideways to see the whole diagram
- 1The browser sends a ClientHello with its key share.
- 2The server replies with its key share and its certificate, signed by a certificate authority.
- 3Both now hold the same session key; the request and response travel encrypted.
Technical deep diveTap to fold
- TLS 1.3
- The current version. One round trip to set up, and zero for a returning visitor with 0-RTT resumption.
- Key exchange
- ECDHE, usually with X25519: each side combines its private number with the other's public share and arrives at the same secret, which never crosses the network.
- Certificate chain
- The site's certificate is signed by an intermediate, which is signed by a root your device already trusts. Let's Encrypt issues most free certificates.
- Symmetric encryption
- After the handshake, fast ciphers such as AES-GCM or ChaCha20-Poly1305 encrypt and authenticate every record.
- What stays visible
- Routers still see source and destination IPs and roughly how much data moves. With Encrypted Client Hello even the site name can be hidden.
Data centres and CDNs
Your request ends at a server in a data centre, or more often at a nearby copy of the site kept by a content delivery network.
In simple words
Websites live on computers called servers, packed into large buildings called data centres with power, cooling and many network links. Because distance costs time, popular sites keep copies of their files in many cities through a CDN. Your request is answered by the closest copy.
Think of it like
A CDN is a chain of local library branches. The main library may be in another country, but popular books are kept on the shelf at your neighbourhood branch. Only rare books are fetched from the main library.
Swipe sideways to see the whole diagram
- 1The edge server answers from its cache in a few ms.
- 2On a cache miss it fetches from the origin once, then serves everyone nearby.
- 3Anycast and DNS send each user to the nearest edge.
Technical deep diveTap to fold
- Data centre
- A building of server racks with redundant power, cooling and fibre from several carriers. Large ones sit next to IXPs and cable landing stations.
- Load balancer
- Spreads incoming connections across many servers and drops unhealthy ones, so the site stays up when a machine fails.
- Reverse proxy
- A server in front of the app, such as nginx, that ends TLS, caches, compresses and forwards requests.
- CDN cache
- Responses are stored according to
Cache-Controlheaders. A cache hit never reaches the origin. - Edge compute
- Small pieces of code that run on the CDN itself, close to users, for redirects, auth checks or personalisation.
Putting it together
Loading one page uses every module above, in order, usually in less than half a second.
In simple words
You tap a link. DNS finds the address, your phone and the server shake hands twice (once to connect, once to encrypt), the server prepares the page, the packets cross towers, fibre and maybe an ocean, get put back in order, and your browser renders the page.
Think of it like
Ordering food: look up the restaurant's number (DNS), call and say hello (TCP), confirm it is really them and agree on a code word (TLS), place the order and wait while they cook (server time), the delivery ride (download), then plating it on your table (render).
Swipe sideways to see the whole diagram
- 1Setup (DNS, TCP, TLS) costs round trips, so it grows with distance.
- 2TTFB, time to first byte, is mostly the server thinking.
- 3Download depends on bandwidth; render on your device.
Technical deep diveTap to fold
- Latency vs bandwidth
- Latency is how long one trip takes; bandwidth is how much fits per second. A wider pipe does not make the trip shorter, so pages with many round trips stay slow on fast but distant links.
- RTT
- Round trip time. Delhi to Mumbai is about 25 ms, Mumbai to Frankfurt about 110 to 140 ms, mostly the speed of light in fibre plus routing detours.
- Connection reuse
- HTTP/2 and HTTP/3 send many requests over one connection, so you pay the handshakes once.
- Measure it
- Browser DevTools, Network tab, Timing shows these exact phases. From a terminal:
curl -w "%{time_namelookup} %{time_connect} %{time_appconnect} %{time_starttransfer}\n" -so /dev/null URL.
Which one does what?
Every part of the journey on one page: the job, the everyday picture, and the technical name.
| Part | Its job | Think of it as | Technical name |
|---|---|---|---|
| Packet | Carries a slice of your data | A numbered envelope | IP packet, TCP segment |
| IP address | Says where a device is | A house address | IPv4, IPv6 |
| DNS | Turns names into addresses | Your contact list | Recursive resolver, authoritative server |
| Cell tower | Radio doorway for phones | The street post box | gNodeB, RAN |
| Mobile core | Checks your SIM, gives you an IP | The main sorting office | 5G core: AMF, SMF, UPF |
| Router | Picks the next hop | A person giving directions | Routing table, BGP, OSPF |
| Modem / ONT | Converts data to line signals | A translator at the door | DOCSIS, VDSL2, GPON |
| Optical fibre | Carries data as light | Morse code with a torch | Single-mode fibre, DWDM |
| Subsea cable | Joins continents | International flight routes | Submarine cable system |
| Landing station | Where cables meet land | The airport | Cable landing station, power feed |
| TCP | Orders packets, refills gaps | Rebuilding a jigsaw | Sequence numbers, ACK, retransmission |
| TLS | Keeps it private | A locked box | TLS 1.3, ECDHE, certificates |
| CDN | Serves a nearby copy | Your local library branch | Edge cache, anycast |
Next time a page loads, picture it: a radio wave to a tower, light through glass, maybe a dive to the ocean floor and back, a few dozen routers pointing the way, and a puzzle put back together, all before you finish blinking twice.