Does a firewall on a machine only block stuff from outside the machine or also from processes on the machine? -

Does a firewall running on the machine only block out content from that machine, or what machine to communicate with Blocks communication between processes through ports?

Specifically, I am writing a windows service that will expose an HTTP permanent service to other processes on the machine. This service will be operational in dynamic / private category - that is, port 80 or not equal. Do I need to worry about a firewall running on the machine?

It will be deployed only on Windows machines - but I think this is a common question.

(Firstly, maybe it's more of a God-centric question)

Firewalls usually block network calls based on protocol (TCP, udp, http, etc), port, and / or IP if you have a local process that is a TCP / IP on your loopback address (127.0.0.1) If the IP calls then yes the firewall can affect the local process.

To be more specific for your question, most firewall programs must be configured to allow only specific addresses and ports and block everything else. So I think you should look at it in your design.