A router is a box with two sides and one job. Something arrives on one side, the router decides where it should go next, and sends it there. Everything else you know it for, handing out addresses, blocking traffic, serving Wi-Fi, is extra work that ended up in the same box.
Two sides
At home, one side is your own network: laptops, phones, a printer, a television. The other side is your provider, and behind that the rest of the internet. The router sits on the border and is the only device that is on both sides at once.
Think of the reception desk of an office building. Everyone inside can walk to the desk, and everything going out or coming in passes it. The desk does not read the letters. It looks at the address on the envelope and decides which corridor it goes down.
A decision per packet
Data on a network does not travel as a file. It travels as packets: small pieces, each with a destination address written on the front. A video call, a download and a doorbell camera all become packets, and they are mixed together on the same cable.
The router looks at the destination address of each packet, on its own, and sends it on. It does not know that twenty thousand of those packets together are a film. That is why one busy download can crowd out a phone call, and why the answer to that is not a faster router but a way of giving some packets priority.
The routing table
The decision comes from a list the router keeps: for this range of addresses, go that way. A small home router has about three lines in it.
- Addresses from 192.168.10.0 to 192.168.10.255 are on my own network, so deliver them directly.
- Addresses from 192.168.20.0 to 192.168.20.255 are my second network, also directly.
- Everything else: hand it to my provider and let them worry about it.
When two lines both fit, the more specific one wins. If you later add a tunnel to the office, a line appears saying "the office network goes through the tunnel". That line is narrower than "everything else", so office traffic takes the tunnel and the rest still goes to your provider. Nothing about the third line had to change. That is the whole trick of routing: narrow rules for the exceptions, one wide rule for the rest.
The default gateway
That wide rule has a name: the default gateway, or the default route. It is the address the router hands a packet to when none of its own networks match.
Your laptop has one too, and it is your router. Your router's default gateway is a machine at your provider. That machine has one as well. The internet is, to a first approximation, a chain of devices each of which only knows its own neighbourhood and where to pass on everything else.
This is also the most common failure you will meet. If the gateway is missing or wrong, everything inside your house still works, printing works, the NAS works, and the internet is dead. It looks like a broken connection and it is a broken sentence in a table.
One public address, many private ones
Your provider gives you one address that the rest of the internet can see. You have more than one device, so the addresses inside your house come from ranges that are reserved for private use and mean nothing on the internet. The router translates between the two, which is why a website sees your household as a single visitor. How that works, and what it costs you, is in NAT and ports. Which private range to pick is in planning your addresses.
Modem, router, switch, access point
Four words for four different jobs. Your provider's white box does three or four of them at once, which is why the words get used as if they were the same thing.
- Modem
- Converts the signal on the line, fibre, coax or copper, into ethernet. It moves bits between two shapes and makes no decisions about addresses at all.
- Router
- Connects two different networks and decides, per packet, which way to send it. This is the device that also does NAT, the firewall and usually DHCP.
- Switch
- Gives you many ports inside one network. It does not look at IP addresses and does not decide routes. See switching and routing.
- Access point
- A switch port turned into radio. It puts wireless devices onto the same network as the cabled ones, and that is all it does. See Wi-Fi as radio.
A MikroTik can be any of these. The same hardware becomes a router, a switch or an access point depending on the configuration you put on it, which is the first question the tool asks you.
Where the tool picks this up
You describe your provider side in WAN and your own side in LAN and DHCP, and the configurator writes the addresses, the default route and the translation between the two for you.