OSI Model - The Internet Is Just Envelopes Thrown Away at Every Router

The wrong mental model

For a long time I pictured a packet as a thing. A little sealed capsule that leaves my laptop, hops from router to router, and arrives at a server on the other side of the planet — the same object the whole way, like a parcel with my address on it.

That picture is wrong. Not slightly wrong. Structurally wrong.

Start at the top

When my browser sends an HTTP request, that request is just data. The layers below it exist to move that data, and each one wraps it in its own header.

• Transport (TCP) wraps it in a segment: source port, destination port, sequence numbers.
• Network (IP) wraps that in a packet: source IP, destination IP, TTL.
• Data link (Ethernet, Wi-Fi) wraps that in a frame: destination MAC first, then source MAC, then a checksum trailer.

Nesting dolls. Fine. That part I had right.

What I had wrong was what happens to those dolls in transit.

Descend: what actually changes per hop

Here's the thing I had to sit with. The destination IP on my packet is the server's IP from the very first instant. Not my router's. The packet is addressed to the far end before it leaves my machine.

But the frame around it? That's addressed to my router's MAC. The frame only knows about the next device on the wire. It's local. It has a range of exactly one hop.

So my router receives the frame, strips it off, and throws it away.

Then it reads the packet inside, decides where to send it next, decrements the TTL, recomputes the header checksum — yes, the IP header is modified at every single router, even when the addresses aren't — and builds a brand new frame addressed to the next hop's MAC.

That happens at every router. Dozens of times. Frame born, frame dead. Every wire gets its own envelope.

One detour the packet can't avoid

The IP addresses do stay constant end-to-end — with one exception, and it happens once, right at my front door.

My home router does NAT: it swaps my private source IP (192.168.x.x) for the public one my ISP assigned, usually rewrites the source port too, and recomputes the TCP checksum because that checksum covers the IPs. After that single rewrite, the addressing is fixed for the rest of the journey.

So the packet isn't immutable either. It's a letter that gets its return address corrected once, and its postmark stamped at every sorting facility.

The click

And then it landed: the packet never travels.

Nothing travels except signals on a wire. What crosses the internet is a pattern — a description of a packet, re-serialized into a fresh frame at every hop, carried one link at a time by envelopes that are shredded on arrival.

My HTTP request doesn't ride across the ocean. It gets rebuilt, hop by hop, thirty times, and the only reason the far end receives "the same" request is that every router faithfully copies the layers above the frame into the next envelope.