If I understand right, applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems. But how does that work without conflicting with other applications such as web-browsers? In fact how do multiple browsers running at once not conflict? Do they all monitor the port and get notified... can you share a port in this way?
I have a feeling this is a dumb question, but not something I ever thought of before, and in other cases I've seen problems when 2 apps are configured to use the same port.
There are 2 ports: a source port (browser) and a destination port (server). The browser asks the OS for an available source port (let's say it receives 33123) then makes a socket connection to the destination port (usually 80/HTTP, 443/HTTPS).
When the web server receives the answer, it sends a response that has 80 as source port and 33123 as destination port.
So if you have 2 browsers concurrently accessing stackoverflow.com, you'd have something like this:
Firefox (localhost:33123) <-----------> stackoverflow.com (69.59.196.211:80)
Chrome (localhost:33124) <-----------> stackoverflow.com (69.59.196.211:80)