demo · v139

Birthday Collision Calculator

Port randomisation is gated on Windows version because Microsoft only widened the ephemeral port range in 2020 H1. With a small range and lots of short-lived TCP connections, the Birthday Problem says you'll re-pick a port still in TIME_WAIT long before you exhaust the range. Move the sliders and see the collision probability and the safety verdict for each Windows era.

simulation

ephemeral port range size 16384 ports

new connections per second 200/s

TIME_WAIT (seconds each port held) 120s

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ports held at steady state

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collision probability (next pick)

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verdict

range usage at steady state

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what the table looks like in real life

Windows era           Default ephemeral range   Randomisation safe?
< Server 2008         1025–5000  (≈4k)         no — too many collisions
Server 2008/Vista     49152–65535 (16k)         maybe — depends on load
Windows 10 (pre-2020) 49152–65535 (16k)         maybe — depends on load
Windows 10 2020 H1+   variable, widened         yes — Chrome 139 enables it

what the table looks like in real life

see also