On Wed, 10 Oct 2018 18:41:13 -0700 (PDT), wrote:

This is a bit of a long shot, but maybe somebody has seen something like this, I am thinking not but ya never know.

Situation is really weird:

My PC can play World of Warships fine, Company of Heroes via Game Ranger fine.

It can browse/websurf the internet fine.

It can even connect to another laptop over same ethernet port by pulling out internet cable modem cable and reconnecting to laptop etc.

But for whatever reason:

If computers from the outside try to connect to my PC it's not working.

Vice versa it's also not working ?!? Maybe cause those devices are natted.

But as far as I know my PC is not NATTED.

Or it's some strange/new ISP kind of NAT that behaves oddly.

Like only big companies/servers are still reachable and not consumer PCs.

As far as I can tell my PC does have a public IP address.

There's no mystery involved when determining whether you have a 'public'
(routable) IP address. It's very straightforward.

1. Open a Command Prompt and run the command "ipconfig". (no quotes)

The output will be similar to this:

C:\Windows\System32ipconfig

Windows IP Configuration

Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::b5fe:220e:f461:d985%20
IPv4 Address. . . . . . . . . . . : 192.168.19.20
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.19.1

You can see that the IPv4 address assigned to this PC is 192.168.19.20

2. Next, in any web browser, navigate to https://www.whatismyip.com/
(There are lots of similar sites, but I suggested that one because it
displays *both* addresses - see below.)
Your local IP address will be displayed, along with your 'Public' IP
address. If the two addresses are the same, there's no NAT. If the two
addresses are different, there is a strong likelihood of NAT, although
not necessarily.

Further, if your local IP address falls within one of the following,
it's NAT because those addresses are not publicly routable on the
Internet:
10.0.0.0 – 10.255.255.255
172.16.0.0 – 172.31.255.255
192.168.0.0 – 192.168.255.255

What kind of electrical defect/or wear and tear could produce such a weird result ?!

So far, you haven't described anything weird or unusual.

(One thing which is on my mind is something different, a hacked or misconfigured/corrupted modem, which somehow blocks this).

What's the make and model of the modem? We can use that info to see
whether it's a cable modem or a DSL modem, and to see whether it's a
plain modem (no router, no firewall, just a simple Ethernet bridge) or
does it have an embedded router (with firewall, port forwarding, etc.)
If it's a plain cable modem, for example, there won't be any
configuration changes available to you, the user.

Also, if it's a cable modem, there's usually a web server located at
http://192.168.100.1
If your modem has such a web server, it's likely that one of the pages
will include your signal levels, (post that info if you have questions),
as well as statistics on "Corrected" signal errors versus
"Uncorrectable" signal errors. Signal levels and error info are
extremely helpful in troubleshooting most issues.

For now I will keep assuming that everything is just fine on my PC and that it's the outside world that simply can't connect to my PC.

Do you *want* the outside world to be able to connect to your computer?
Most people don't.

Also is there any service out there on the internet where there is a public computer that can send some traffic to a specific tcp or udp port ?!

I second Paul's suggestion to use Steve Gibson's Shields Up! program to
scan your ports, if you're interested in seeing what's open versus
closed versus stealth. Ideally, everything is 'stealth' unless you have
a specific reason for 'open' or 'closed'.