To check specific port
======================
# lsof -i tcp: to display hostnames
or
# lsof -ni tcp: to display IP address only
To check all ports
==================
# lsof -ni udp
# lsof -ni tcp
To check a specific IP or hostname and port
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# lsof -ni [protocol][@hostname|hostaddr][:service|port]
To list open files (file descriptors) by a process
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# lsof -p