Computed here

• Part 2 · Cables and waves

  5The 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

  7Wi-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

  8Cellular 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

  9Topology — 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

  14Routing — 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 7 · Boundaries and filtering

  26Firewalls — 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

  27Stateful 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

  28Switching — 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

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