How do Layer 2 (MAC) and Layer 3 (IP) addresses behave differently when sending to a remote network?
Follow one packet across a multi-hop path and watch the headers: the IP addresses never change, but the MAC addresses are completely rewritten every time the frame crosses a router.
* IP stays fixed end-to-end; the destination MAC is rewritten at every hop. *
When a host sends to a remote network, the data travels through a chain of links. Picture the path source host → Router 1 → Router 2 → destination server, where each arrow is one "hop" (one physical link). At each hop the frame is decapsulated, the router decides where to send it next, and a brand-new frame is built.
The reason the two address types behave so differently is scope:
- Layer 3 (IP) is end-to-end (global). The IP header carries the original sender's address and the final destination's address, and a router never rewrites them — it only reads the destination IP to look up the next hop. So from the first link to the last, source IP and destination IP are identical. They are the "from / to" on the envelope that everyone along the way must respect.
- Layer 2 (MAC) is link-local (one hop only). A MAC address only has meaning on a single shared link, so it can only ever name "the device at the other end of this cable." Each time a router forwards the packet it strips the old frame and writes a new one whose source MAC is its own outgoing interface and whose destination MAC is the next device on that link.
Tracing the destination MAC down the example path makes the rewriting concrete:
| Hop (one link) | Source MAC | Destination MAC |
|---|---|---|
| host → Router 1 | host's NIC | Router 1's incoming interface |
| Router 1 → Router 2 | Router 1's outgoing interface | Router 2's incoming interface |
| Router 2 → server | Router 2's outgoing interface | server's NIC |
Gotcha: the destination MAC is never the final server's MAC until the very last hop — on every earlier link it points at the next router, because the sender cannot reach a device it has no direct link to. That is exactly why a host needs a default gateway to talk off-network.
Go deeper:
Host to Host through a Router — Practical Networking — the canonical "L3 header stays, L2 header regenerated each hop" walk-through.- MAC address — why a MAC only has meaning on a single link.