I have alot of questions on my mind right now, but I'll only ask the ones that are particularily intriguing me.
First off: - I port-scanned my Public IPV4 address; and it returned a few ports being 'open' and some of them 'filtered' (as you would expect), but I then tried to scan my mate's IPV4 address, who lives a couple of streets away from me (yes, I already made sure that he is okay with this and that I am only doing this for my personal curiosity), and it showed all ports that I scanned were closed and/or filtered. I tried a large number of flags; fragmentation; really slow scan times and types; but every time it would show that all of the scanned ports are closed or filtered. I looked closer and saw that the reason it showed these results were because of there being 'no-respone'. Would this be the firewall/IDS blocking(dropping) the packets? I have trouble thinking so because wouldn't the router atleast reply to something like an Echo request on port 7?
2: -(Pardon my inexperience here)I also port-scanned a single port, 80 on his IPV4 address, it sent me a ICMP Type3/Code13 (Communication administratively prohibited). But the interesting aspect of it was that the reply came from a different IP address, is there an explanation to this? (I had several theories myself but decided to come here for an expert opinion)
3: -Regarding question 2 above, why isn't port 80 always open? If the host is watching a video or listening to some music-streaming website, shouldn't port 80 come up as open? Is this because the port sees that it's not a HTTP oriented packet and automatically blocks it?
I would also love if someone could point me to some links explaining their answer so I can expand my knowledge further and perhaps answer some of my questions by myself.