How and Why Your Emails Get to Someone Else's Computer

It’s all about addressing and delivery.

A computer communicates with a network interface device such as a network adapter card. The network interface device has a unique physical address and is designed to receive data sent to that physical address, which is burned into the card when it is manufactured.

A device such as an Ethernet card does not know any of the details of the protocol layers. It does not know its computer’s IP address or where data that comes its way (a “frame”) is being sent. It just “listens” to the incoming data packet, waits for one that is addressed to its own physical address, and then passes that frame up the TCP/IP layer stack.

The brains of the outfit.

Your networking card is just waiting for its orders. TCP/IP makes it all work out by organizing the network around a logical, hierarchical addressing scheme. This “logical addressing” is maintained by the IP protocol at the Internet layer.

The logical address is called the IP address. Another Internet layer protocol called the Address Resolution Protocol (ARP) puts together a table that maps IP addresses to physical addresses, or an actual piece (or segment) of networking hardware. This ARP table is the link between the IP address and the physical address burned into the network adapter card.

Here’s how the TCP/IP software coordinates sending data over networks…and how your email gets to your friend across the country or on the next floor at your office:

If the data you’re sending to a destination IP address (another computer) is on the same network you’re on, then your computer (the source) sends the data packet directly to the destination. The IP address is resolved to a physical address using ARP and the data is sent to the other guy’s computer network adapter.

If the destination for your email or inquiry is on a different network from the one your computer is on (to someone out there in the online or Internet world), the following happens:

Your message or data is directed to a gateway or router. Your router is the key part of your network that can forward data to other networks…anywhere in the world, in fact.
The data is routed through the gateway to a higher-level network. The process is repeated over and over, gateway by gateway, until it reaches its destination address—that could be a website or someone’s email account. They are all identified ultimately by their IP addresses.
The datagram (basic transfer unit) passes through these chains of gateways to the destination network first. There, the destination IP address is mapped/matched to a physical address using ARP, and the data is directed to the destination network adapter.

When that happens, your email pops up on your friend’s computer screen. And all this information about gateways, network segments, and ARP fades away…until they get ready to send you an email. And then all of the pieces of the Internet protocols go back to work again.