NETKUBE LAB · NETWORKING
Network Fundamental
Grouped into parts, in the order they are meant to be read — or take only what you came for
29 lessons · free to read · 419 min read
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1 Addresses and identity
The numbers that say who is who, and how far each one can be believed
- 1Internet Protocol — the layer that promises you nothing
What IP is, what is inside a packet header, how a machine decides where to send, and why a protocol that guarantees nothing became the foundation of the entire internet
- 2IPv4 subnetting — the bit boundary everyone recites and few understand
How a subnet mask works at the bit level, why it must be a boundary rather than a range of numbers, where it came from in 1985, and the cases where the usual rule about first and last addresses does not hold
- 3MAC address — what is it actually for
A MAC address has one job, carrying a frame across a single link. It does not travel past a router, it is no longer a reliable device identifier, and it was never a security measure
2 Cables and waves
What the signal travels on, how fast it can go, and where the ceiling comes from
- 4The physical layer — there are no bits on the wire
Why encoding has to exist, why the pairs are twisted, why it stops at a hundred metres, what is in the auto-negotiation base page, and why duplex mismatch is the quietest failure this layer has
- 5The ceiling of a channel — a number nobody gets past
In 1948 someone proved that every channel has a ceiling computable from its width and its noise, and that no coding scheme can ever cross it
- 6Ethernet — the network designed to collide, and it beat everything else
Where Ethernet came from, from radio in Hawaii to the memo of 22 May 1973 that named it, and the engineering behind the numbers 64, 1500 and 1536 that are still on your machine today
- 7Wi-Fi — a radio that cannot hear itself
Everything that separates Wi-Fi from Ethernet follows from one fact of physics, and the measured speed on a laptop that never moved swung by almost seven times
- 8Cellular networks — the invention was not mobility
Mobile radiotelephones existed thirty years before cellular. What cellular added was reusing the same frequencies, and that is pure geometry you can compute
3 Inside one network
When every machine shares one network, what the switch has to deal with
- 9Topology — the drawing and the reality
Textbooks draw bus, ring, star and mesh, but every network you buy today is physically a star, and how it behaves depends on the box in the middle
- 10VLAN — the four bytes that split one switch into many
How VLANs work at the bit level, why the tag is only four bytes, the difference between access and trunk ports, and why the native VLAN is the quietest mistake in networking
- 11Spanning Tree — the protocol that fixes a problem Ethernet created
Why one redundant cable can take down a whole network, how Radia Perlman solved it in 1985, what is in the 35-byte BPDU, and every step of the development from 802.1D to what replaced it
- 12EtherChannel and LACP — four cables is not four times faster
Bundling links does not make one connection faster; it adds lanes. And LACP exists because a cable can lie to you while its light stays green
- 13ARP — the seam the whole stack rests its trust on
The question ARP actually asks is not the one people think, a cache rule from 1982 that is still in force, why every packet is nearly a third empty padding, and why there is nothing at all to verify
4 Across networks
Once you leave your own network, who decides, and what is guaranteed
- 14Routing — what the gateway actually does next
Nineteen articles on this site mention routes, and none explained how a router decides. This one answers with a single rule you can measure on your own machine
- 15TCP and UDP — two answers to the question IP does not answer
What is in the eight-byte and twenty-byte headers, why the handshake takes three messages, the two windows people most often confuse, and why TIME_WAIT on a real machine does not match what the RFC says
- 16Protocols and their layers — seven that nobody uses in full
The seven-layer model is a map, not the road. The standard itself says the split cannot be proven best, and TCP breaks its rules in the very first line of the checksum formula
- 17Error detection — a checksum that is weak on purpose
The internet checksum cannot detect some kinds of corruption at all — not because of a bug, but because of a property a 1978 design document lists as an advantage
5 Diagnostic tools
Two commands everyone can type, and the results most often misread
- 18ping — the forty-year-old command still typed every day
The history, the mechanism and the output of ping, from the first line for someone who has never used it to the reasons a network engineer should not trust all of it
- 19traceroute — why it does not show you the path you think it does
How traceroute and tracert work, where they came from, how to read their output, and why the list they print is not the route your packets actually travelled
6 The services that make it usable
What has to work before you can open a single page
- 20DHCP — to ask for an address you need an address
The chicken-and-egg problem an RFC posed in 1985, why it takes four messages, why 81 percent of the header is still legacy nobody uses, and reading a real packet off your own machine
- 21DNS — why "it's always DNS" is right often enough to be a joke
How DNS works, where it came from, and how to diagnose it — from the single HOSTS.TXT file that once held the whole internet to the reasons two people asking the same name get different answers
- 22HTTP — a protocol that promises very little
What a request and a response actually look like, where the body ends, how two ends can read the same message differently, and the effect of caching measured in real bytes from this site
- 23TLS — HTTPS does not mean safe
The three promises the RFC makes and the four people think it makes, this site's real certificate chain, the name check that carries the whole thing alone, and what stays visible even once encrypted
- 24Certificate lifetimes — why only 47 days
47 days is not a tightening of the screws. It is an admission that revocation never worked — and the 200-day ceiling has been in force since March 2026
7 Boundaries and filtering
Who gets in, who does not, and how the device can tell
- 25NAT — why the internet runs on something meant to be temporary
Why the whole internet depends on NAT, how the translation table works, why nobody outside can connect in, and why it is not a firewall
- 26Firewalls — the last matching rule wins
A firewall does not decide what is safe, it decides what matches a rule — and the packet filter shipping with macOS defaults to pass, not block
- 27Stateful and stateless — what a firewall remembers, and what remembering costs
A stateful firewall runs its own state machine alongside the endpoints, invents a connection for UDP where none exists, and pays three prices for the privilege
8 Out to the wider world
Long distances, and the set of addresses that is running out
- 28Switching — why the internet won
A network has two choices, reserve the path first or send piece by piece and find the way — and the ones that lost did not lose by being worse, while ATM paid for its 53-byte cell forever
- 29IPv6 — not IPv4 with longer addresses
A header twice the size that makes routers do less work, the four things that were deleted, why broadcast disappeared entirely, how a machine finds its own address, and why ICMPv6 cannot be switched off
Or read by theme instead
Three threads the reading order cannot show, because they cut across the parts
- Trust
Mechanisms that work by believing an answer, with nothing to prove the answer is honest
- Measured on a real machine
Carries measurements from a real machine, with the commands to reproduce them
- Computed here
Carries computed results with the working and the starting numbers shown