Router without DHCP set and has external IP from local perspective

Question

I have a network like: x.x.0.1 - ISP's AP, then mikrotik router x.x.0.5 -> set up by specialist and another router x.x.0.7 -> for my learning purposes.

My problem is, that I really don't understand one of the x.x.0.5 (the specialist made) settings and I would like to know how this works:

Here's a screenshot:

This router makes a subnet x.x.10.0/24 and it's name is x.x.10.1.

What I don't get is the Local Network tab.

a) The IP address entered there is my external IP and when I type it from within the network I get to my router's page ( I can also get to router page with x.x.10.1, but it's stated nowhere in the config gui (but it's standard way how I do it) ) and from the outer world it port forwards me to my server and http website.

b) The DHCP server checkbox is unchecked, but from my experience, if I do that on the other (my mikrotik for training) router I got kicked off internet and my router gets unreachable so I have to reset it. The other wierd thing is that in mikrotik GUI (on the one that I posted screen of), there obviously must run DHCP server, because on it`s tab I can see all leases and manage pools and other things.

However, everything works fine and I'm just anxious about how does it work. Can please anyone explain?

Answer 1

A. Upon receiving a request destined to it on port 80, your router checks to see if the sender (source IP address) is from a local subnet. If yes, it passes the request to a local process and gives you the Web-based administration page. If not, it checks to see if other conditions are met (including whether a port-forwarding or DMZ rule exists). In your case, there is a port-forwarding rule for port 80, so it forwards the incoming request to a host you specified.

B. Most likely your AP is serving the DHCP requests.

Answer 2

I see that your external IP address is a private (RFC1918) address, which means there's some NAT involved on your ISP's side. (Some ISPs have more customers than public IP addresses, so they need to resort to NAT). To find out, visit a site like https://www.whatismyip.com/ – this will give you the public IP under which you are visible on the Internet.

The internal IP is interesting – in fact, it is not an RFC1918 address but an "official" one. Have you by any chance requested a feature such as "static IP" or "public IP" from your ISP? Also, does this address match the one reported by https://www.whatismyip.com/? If that is the case, then your ISP might have a reverse NAT policy in place which maps 79.98.*.47 back to 192.168.0.5 (the IP of your router's external IF), which is how outside requests reach your router.

As for the DHCP requests, it looks like your training router is answering the DHCP requests. DHCP works through broadcasts – the client sends out a "give me an address" broadcast and any DHCP server which hears it will answer it. If the client is on a physical network which can talk to both routers and the training router has DHCP enabled on that interface, it will offer the client a DHCP lease and the client will take it. Internet access will work as long as the DHCP lease specifies a valid route to the Internet, i.e. either has the main router as its default gateway or the training router is correctly set up for Internet access and specified as the default gateway.

Test: if you switch off the training router, will you still get Internet access? (Do this from a computer that uses DHCP, disconnect from the network once and reconnect to ensure you don't continue on an old DHCP lease that you got from the training router before you switched it off.)