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
Other good answers here but will add more detail and something I’m not seeing mentioned elsewhere.
Network communication operates in several distinct layers (See: [The OSI Model](https://s7280.pcdn.co/wp-content/uploads/2018/06/osi-model-7-layers-1.png)) – Your router is basically just concerned with the Physical (physical cabling), data link (Raw network protocol, almost always Ethernet), Network (IP data packets and routing), and Transport (TCP, UDP packets) layers. It exists to route packets on a local network consisting of any number of devices to and from where they want to go, and gatekeep anything that shouldn’t be happening with a firewall.
In order to actually download something or otherwise interact with you in a meaningful way, your computer is also handling things (in addition to the above layers) on the Session (OS-side Sockets/port sessions), Presentation (OS-level usage of specific protocols such as IMAP for mail), and Application (Application-specific usage of Presentation layers).
Normally when you download mail to say, outlook – Outlook the application reaches out at the application and presentation layers, the OS gets that request and opens up a network socket on the Session layer, the for example IMAP packet will be looped into a TCP packet on the transport layer and sent out through an ethernet frame on the data link layer, then out the physical cable on the physical layer to your router which then needs to route it out to your modem and to the mail server. The mail server then responds with whatever you ask for, including a file download. The mail server isn’t so much pushing a file to you, as you’re asking for a file and it’s sending it to you.
In order for someone to send a file to you from an outside network, highly specific conditions would be required on both your router and computer across all layers. The router would need to accept unsolicited inbound requests, allow it to be routed to a specific computer/know where to route it to, and on the specific port being used. The computer then would need to also be accepting those same requests and ports, allow that to be passed to a specific application or service on the local computer, and that specific application or service would need to accept that unsolicited file.
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