Linux - How to Open a Port

Hi,

I would like to open a specific port for use with a bespoke application.

Before everyone points me to other threads - I read a few but couldn't find any specific to my problem.

Iptables / firewall is disabled
SELinux is also disabled

I would just like to assign this port to this application?

i have tried the commands to add iptables ports and adding a line to the iptables config file but without any success.

RHEL5.6

can anyone advise what I am doing wrong?