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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
Status
Info
Address Port
Port Counter
Address Counter
Port Min/Max/Delta Assumed Start/Width Reusage
Histogram 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
2D Source Port distribution 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:

A sequence of requests are made by browser using 'XMLHttpRequest' to a page returning client IP and port for IPv4 and IPv6
Relation of delta of seen min/max port to amount of ports plus distribution and histogram will show whether Carrier-Grade NAT is probably active or not
Examples:

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:

as user "root" execute (don't forget to reset after the test...) for IPv4+IPv6
# record current value sysctl net.ipv4.ip_local_port_range net.ipv4.ip_local_port_range = 32768 60999 # set small range and run test sysctl -w net.ipv4.ip_local_port_range="32768 33023" # reset back to original sysctl -w net.ipv4.ip_local_port_range="32768 60999"
as user "root" execute (don't forget to reset after the test...) for IPv4-only
# add dedicated IPv4 Source-NAT rule restricting outgoing ports iptables -t nat -I POSTROUTING -p tcp -j SNAT --to-source :41984-42239 # run test # remove NAT rule iptables -t nat -D POSTROUTING -p tcp -j SNAT --to-source :41984-42239
as user "root" execute (don't forget to reset after the test...) for IPv6-only
# add dedicated IPv6 Source-NAT rule restricting outgoing ports ip6tables -t nat -I POSTROUTING -p tcp -j SNAT --to-source :41984-42239 # run test # remove NAT rule ip6tables -t nat -D POSTROUTING -p tcp -j SNAT --to-source :41984-42239