On a home private network, the router would have a private IP address something like this 192.168.0.1. On the home network there are 5 computers. Each has a local IP address 192.168.0.xx e.g. 192.168.0.2 – 192.168.0.6. But the computers themselves do not have a public IP, only the router does. So would it be possible for someone from an outside network to send a file to one specific computer in my home network? Like if they know the MAC address of the device.
And how is it that emails can reach e.g. outlook application installed on one computer with one IP address, but still be accessed from another computer with a completely different IP address.
In: 5
Basically speaking, your home router functions like a mail room at an office building. All messages and packages in and out of that building pass through that mail room before being sent to the recipients. This room knows about both the outside world (public IPs) and the people throughout the building (Devices with the 192.168.x.x. IPs that are not valid on the Internet).
This metaphorical room dominantly relies upon a system of rules that depends on the way the package is marked (source and destination port numbers + source and destination IP addresses). When someone within the building sends a message out to the world asking someone outside for something like a web page, the mail room makes a note in a ledger (called a “sessions table” in the router) that records the markings, then changes the package’s source address to the building’s public address (this is called “network address translation”). This says that there is a reply expected.
When one or more packages like that web page arrives as a reply to the building’s public address, the mail room goes over the ledger and sees who was expecting it with that arrangement and sends the packages to the designated internal recipient. If there is no entry in the ledger, the mail room doesn’t know how to deliver it, it throws the packages in the garbage and depending on policy, tells the sender it was thrown in the garbage (“connection refused”).
To keep the ledger light, when an entry in the ledger is unused for a time, the mail room strikes it out.
In this metaphorical world, email is like a PO Box. That PO Box exists at an outside post office represented by your email provider’s servers. When someone in that building accesses their email, they basically are sending a request to the post office asking for the post office to deliver a package with everything in the PO Box via the building’s ‘mail room’ according to these same rules.
There is are special occasions (like port forwarding) where a special note is made on the wall of the mail room saying anything received with a specific number written on it (i.e. destination port number) to the building’s public street address, it is directed to a pre-arranged internal party. This can be for anything from a web server (port 80, 443) or a game server.
Latest Answers