Computed here

• Part 1 · Addresses and identity

  3Subnet arithmetic by hand — six questions with one answer

  2026-09-20 · 18 min read

  Find the network ID, subnet mask, broadcast, host count, host range and next network with a single method, then go on to subnetting, route summarization and VLSM, with five exercises whose answers open on a click

  Computed hereMeasured on a real machine

• Part 1 · Addresses and identity

  5Multicast — an address that refers to no machine at all

  2026-09-17 · 6 min read

  The machine you are on has already joined several multicast groups without being told, and the address mapping throws five bits away

  Measured on a real machineComputed here

• Part 2 · Cables and waves

  7The ceiling of a channel — a number nobody gets past

  2026-09-08 · 8 min read

  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

  Computed here

• Part 2 · Cables and waves

  9Wi-Fi — a radio that cannot hear itself

  2026-09-05 · 11 min read

  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

  Measured on a real machineComputed here

• Part 2 · Cables and waves

  10Cellular networks — the invention was not mobility

  2026-09-09 · 8 min read

  Mobile radiotelephones existed thirty years before cellular. What cellular added was reusing the same frequencies, and that is pure geometry you can compute

  Computed here

• Part 3 · Inside one network

  11Collision and broadcast domains — the line everyone can name and few can place

  2026-09-15 · 10 min read

  What each device actually stops, and thirty years in which one boundary shrank to nothing while the other never moved at all

  Measured on a real machineComputed here

• Part 3 · Inside one network

  12Topology — the drawing and the reality

  2026-09-10 · 8 min read

  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

  Computed here

• Part 4 · Across networks

  17Routing — what the gateway actually does next

  2026-09-11 · 9 min read

  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

  TrustMeasured on a real machineComputed here

• Part 4 · Across networks

  18MTU — the number nobody sets, and everybody has to guess right

  2026-09-16 · 8 min read

  Eight articles on this site mention MTU and none explains it. This one finds the edge with a single command, then shows why the mechanism that hunts for it breaks so often

  Measured on a real machineComputed here

• Part 7 · Boundaries and filtering

  31Firewalls — the last matching rule wins

  2026-09-06 · 10 min read

  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

  Measured on a real machineComputed here

• Part 7 · Boundaries and filtering

  32Stateful and stateless — what a firewall remembers, and what remembering costs

  2026-09-12 · 10 min read

  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

  Measured on a real machineComputed here

• Part 8 · Out to the wider world

  33BGP — the internet works because everyone believes the neighbours

  2026-09-14 · 9 min read

  The routing article ended saying nothing proves the neighbour is telling the truth. This one shows that BGP's own standards documents say exactly that

  TrustMeasured on a real machineComputed here

• Part 8 · Out to the wider world

  34Switching — why the internet won

  2026-09-07 · 9 min read

  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

  Computed here

• Part 8 · Out to the wider world

  36VPNs and tunnels — a packet wrapped inside a packet

  2026-09-18 · 6 min read

  The machine this was written on has fifteen tunnel interfaces set to five different MTUs, and one of them is larger than the wire it rides on

  Measured on a real machineComputed here

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