Can a webpage tell the source IP of a message? (no-ip.com like appliction)

I'm writing an application like no-ip.com , the user will send a _POST
message includes the domain name to the website, and the website
should map the domain name in the message with the source IP of the
message. and update it in the database.

so when someone have a computer at home with dynamic IP and he want to
access it remotely, he can always know his new IP from this website.

is it possible for a Webpage installed on a webhost server to
determine the source IP of a packet?

any advise from where should i start? where should i search about the
subject?

thanks alot !

* i know how to do it if it's on my computer, i'll just make some C
script to capture the packets and tell. but first, that's an over
kill, second, it doesn't work in this case, i'm use a webhost server,
not my own.
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