How to check if port is in use on Linux or Unix

How do I determine if a port is in use under Linux or Unix-like system? How can I verify which ports are listening on Linux server? How do I check if port is in use on Linux operating system using the CLI?
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Linux: Check Network Connection Command

How do I check network connections under Linux using command line options?
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ping command: TTL Expired In Transit Error and Solution

When I try to ping other UNIX / Linux / BSD or MS-Windows servers in our network I get the following message:

TTL Expired In Transit

How do I fix this problem?
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CentOS / Redhat Linux: Install OpenNTPD To Synchronize The Local Clock

OpenNTPD is a Unix system daemon implementing the Network Time Protocol to synchronize the local clock of a computer system with remote NTP servers. How do I install OpenNTPD under CentOS / RHEL / Fedora Linux instead of default NTPD client / server supplied by Linux operating system?
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