OpenBSD routing - detects WAN IP links back to firewall?

Hello,

So yesterday I was working on my BSD router, I realised that if I
nmap'ed my external IP from my internal IP, it would treat it still as
an internal request.

Does OpenBSD detect when a packet endpoint is designated for it, for
example with traditional ISP routers, if you send a packet to their WAN
address, you would see the following:

My laptop (192.168.0.104) --> ISP router (192.168.0.1) - NAT'd -> ISP
--> ISP router

Now when I was setting up my OpenBSD router, I believe the following was
occurring (hence why the nmap preserved permissions set for internal IPs
and not external).

My laptop (192.168.2.3) --> OpenBSD Router

Even though I was referencing the BSD router WAN address and not its
WAN, I have my internal DNS server blocked externally, one because it is
for internal use to allow mapping of hostnames to LAN addresses instead
of resolving their WAN address.

But despite using the public IP address of the BSD router, it still
seemed to detect that the packet was for it, and I am not too sure why?

Of course this behaviour would be preferred, less packets going to ISP
and back again saves me resources (and money, bandwidth is not cheap),
but I am curious to see if my prediction of this behaviour is true, and
or is default, and why it occurs.

If anyone could explain to me their thoughts about this, or how it
works, it would be appreciated.

Thank you,
--
Polarian
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