Talking to a foreign network: the default gateway

When the destination is on a different network, a host cannot reach it directly — and does not even try. It hands the packet to one door out of its network, the default gateway, and lets the router take it from there.

Same start, different answer

The previous article established a host's very first move: it has data, it knows the destination IP, and it checks its subnet mask to decide whether the destination is local or foreign. Last time the answer was local, and host A reached host B directly.

This time host A wants to reach host C, and the subnet-mask check comes out the other way — host C is on a foreign network.

That single difference changes everything that follows.

The L3 header is the same; the L2 header is the problem

Host A still builds the Layer 3 header exactly as before: source is host A, destination is host C's IP. That is end-to-end delivery, and it does not change — the final destination is host C no matter how many networks lie between them.

The Layer 2 header is where it breaks. Layer 2 is hop-to-hop, and host C is not the next hop — it is not directly reachable at all. So whose MAC goes in the L2 header? Not host C's. The next hop is the router.

This is the concrete payoff of the MAC-versus-IP split from the OSI article. The Layer 3 destination is the final host (host C), while the Layer 2 destination is only ever the next device on the path — and here that next device is the router, not the destination.

Why host A cannot simply ARP for host C

The obvious question: why not just ARP for host C directly, the way host B was resolved last time? Because ARP works by broadcast, and broadcasts do not cross routers — a router breaks up broadcast domains, exactly as the hubs article foreshadowed. Host A's ARP request for host C would never leave the local network, so host C would never hear it and never reply.

Host A therefore cannot resolve a MAC on a different network. That is not a limitation to work around — it is the entire reason default gateways exist. Foreign traffic has to be handed to something that can reach beyond the local network, and that something is the router.

The default gateway: the one door out

To reach a foreign network, host A sends the frame to its router, and the router's IP is stored on host A as its default gateway. (The router's near-side interface sits inside host A's own network — it shares the subnet — which is exactly why host A can ARP for it locally even though it leads everywhere else.)

This is why connecting a computer to a network configures three things, not one — and each now has an obvious job:

• IP address — host A's own identity.
• Subnet mask — how host A decides whether a destination is local or foreign.
• Default gateway — where host A sends anything that turns out to be foreign.

The ipconfig (or ip addr) output shows exactly these three, and the reason for the trio is precisely the decision this article is built around.

ARP — but for the router this time

Host A knows the gateway's IP but not its MAC, so it performs the same ARP exchange as before, with one change: the target is the router, not host C. It broadcasts an ARP request for the gateway's IP, the router replies with its MAC, and host A caches the mapping.

Now host A completes the Layer 2 header with the router's MAC as the destination — wrapped around a packet whose Layer 3 destination is still host C.

It sends the frame to the router. The router receives it, strips the Layer 2 header, and takes over the job of moving the packet closer to host C — the start of routing, which is a whole chapter ahead. Host A's part is finished.

One gateway MAC serves every foreign host

There is a neat efficiency here. Host A only ever needs the router's MAC once. From host A's point of view, the entire rest of the internet is reachable through a single door — the router — so the same gateway ARP entry is reused for every foreign destination it will ever contact.

The universal first step

Both articles now collapse into one rule that a host applies to every packet it ever sends:

host A has data for some destination IP:

  is the destination on my network?   (check with the subnet mask)
      |
      +-- LOCAL    -> ARP for the destination's own MAC, send directly
      |
      +-- FOREIGN  -> ARP for the default gateway's MAC, send to the router

Local or foreign, the Layer 3 header always names the final destination; only the Layer 2 header — the next-hop MAC — differs between the two paths.

Host A has now handed a foreign-bound frame to the router. What the router does with it — how it decides where to send it next, and how this ARP-and-forward dance repeats hop by hop across the internet — is the subject of the router chapters, which the coming articles begin.