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Carrier-Grade NAT (CGN) Test Page


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In case you see only (parts of) one square filling in the IPv4 2D source port distribution and dots are changing colors to yellow/red, yellow/red lines in the histogram and the port reusage > 0%, your system is probably behind a Carrier-Grade NAT device.
Update of graphics every ms and status every ms) Unexpected behavior may occur when used via proxy
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Control Amount of connections to use for test:

for test use 8, for analysis 1024/2048 is a good choice

Workflow step
Time [s] Consumed/Left
Protocol IPv4 IPv6
Port Counter
Address Counter
Port Min/Max/Delta
Assumed Start/Width
each x covers 256 ports
each y-dot covers up to 16 hits
and above 256 hits as y-dot
a line shows an overflow
Source Port

each dot refers to
one source port
same port used a 2nd time
same port used >= 3 times
How it works using Javascript in local browser: Using Linux you can simulate the behavior of CGN using e.g. on the host itself by reducing source port range to 256 ports only: See also
(P) & (C) 2013-2024 by Peter Bieringer <> / Credits to Robert Eisele for pnglib.js
2024-02-17, webmaster at bieringer dot de

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