Learning Center

Six short lessons for a complete beginner: what happens, one step at a time, when you open a website.

1. 1What happens when you open a website?Your browser finds the site, connects to it, locks the connection, and only then asks for the page.5 min
2. 2Your devices are on a networkLaptops, phones, a switch and one router: what a home network is made of, and where it meets the internet.5 min
3. 3Messages travel in small packetsEverything you send is cut into packets, and routers pass each one on, one hop at a time.5 min
4. 4Finding a website's addressComputers need a number, not a name. A resolver finds it by asking in steps, and writes the answer down.5 min
5. 5Asking for the pageThe browser sends a request, the server sends a response, and a status code says how it went.5 min
6. 6Keeping it private with HTTPSA handshake agrees a secret and checks the site's ID. After that, only the two ends can read what is sent.5 min

What a network actually is, how machines are addressed, and how packets move.

1. 1What a network actually isTwo machines, one link, and the smallest possible agreement about how to take turns.6 min
2. 2Addresses, subnets, and the default gatewayWhy your laptop has a number nobody else can reach, and how it decides whether to shout or to ask the router.8 min
3. 3Routing: how a packet finds its wayNobody knows the whole route. Every router knows one thing -- which way is closer -- and that is enough.9 min
4. 4TCP and UDP: two ways to use a networkOne transport rebuilds reliability out of unreliable parts. The other hands you the parts.9 min

One URL, end to end: DNS, then TCP, then TLS, then HTTP, then pixels.

1. 1From a URL to pixelsFive stages between pressing Enter and seeing anything, and the request is only the fourth of them.9 min
2. 2The second visitThe same page again, minutes later. Four different mechanisms each remove a different part of the cost.8 min
3. 3Why pages feel slowA fast link on the far side of the planet is still slow, and no amount of bandwidth fixes it.8 min
4. 4When a page load failsThree failures that look identical in a browser and are nothing alike underneath -- told apart by how long they take.8 min

DNS in depth -- record types, the resolver hierarchy, caching, and DNSSEC.

1. 1How a name is resolvedNobody holds a list of every domain. The answer is found by asking three servers, none of which knows it.9 min
2. 2Records, aliases, and delegationA zone is a piece of the tree somebody was handed. A CNAME is how they hand part of it on again.8 min
3. 3Caching, TTLs, and saying noAlmost every DNS query in the world is answered from memory. This is the lesson about the ones that are not.8 min
4. 4DNSSEC and the chain of trustDNS answers arrive unauthenticated. DNSSEC does not encrypt them -- it lets you prove they were not edited.9 min

HTTP semantics: methods, status codes, caching, cookies, sessions, and CORS.

1. 1The anatomy of an HTTP messageA request is a line, some fields, a blank line, and maybe a body. That blank line does more work than it looks like.8 min
2. 2Status codes and redirectsThree digits that tell a client what to do next -- and four redirect codes that disagree about what to do with a POST.7 min
3. 3HTTP cachingThe fastest request is the one never sent. The second fastest comes back with no body at all.9 min
4. 4Cookies and sessionsHTTP forgets you between requests. A session is a value the server asks the browser to keep repeating.9 min
5. 5Origins, CORS, and the preflightThe browser sends the request anyway. What it withholds is the answer -- and that distinction is the whole model.8 min
6. 6HTTP/1.1, HTTP/2, HTTP/3Three versions of one set of semantics, differing only in how bytes are framed -- each fixing the queue the last one left.9 min

TLS and certificates, and the sharp line between what HTTPS protects and what it does not.

1. 1What TLS actually doesThree guarantees, negotiated in one round trip, over a wire the attacker is reading the whole time.9 min
2. 2Certificates, and where trust comes fromA certificate is a signed claim about a name. Three separate checks can fail it, and the browser says so differently each time.9 min
3. 3Handshakes, versions, and resumptionTLS 1.2 needed two round trips and told the world who you were talking to. TLS 1.3 needs one, or none.9 min
4. 4What HTTPS does not protectThe padlock is a statement about a name and a key. It says nothing about who holds them or what they do with your data.8 min

REST, GraphQL, webhooks, WebSockets and SSE -- and when each one is the right shape.

1. 1REST, and what GraphQL changesNouns in the URL, verbs in the method -- and the one query language that throws that arrangement out.9 min
2. 2API authenticationA key says who you are. A token says who you are and what you may do. Neither is a secret once it is in a URL.9 min
3. 3Pagination and rate limitsTwo problems that only appear at scale: a list that changes while you read it, and a client that wants everything at once.9 min
4. 4Webhooks: when the API calls youReverse the arrow and every assumption changes. Your endpoint is now a server, and it will be called twice.8 min
5. 5Choosing a real-time transportPolling, long polling, SSE and WebSocket, priced in requests and bytes over the same sixty seconds.8 min
6. 6The upgrade, and the frames after itIt starts as an ordinary GET. One blank line later HTTP is gone, and the same socket carries frames both ways.9 min
7. 7Keeping a WebSocket aliveAn open socket is not a working one. Three things go wrong here, and two of them are silent.8 min

The machines between you and the origin: CDNs, load balancers, proxies, NAT, firewalls.

1. 1CDNs and the edgeA shared HTTP cache a few milliseconds away, obeying the same rules you already know from the browser cache.8 min
2. 2Load balancers and reverse proxiesOne hostname, a dozen machines. Everything between the edge and the database, and why each layer is there.9 min
3. 3NAT and private addressesThere were never enough IPv4 addresses. NAT is the workaround almost every home network on Earth runs on.8 min
4. 4Firewalls and segmentationA firewall is a policy at a boundary. The interesting decision is where you put the boundary.8 min