Routers forward packets hop-by-hop using routing tables: each entry is a prefix, next-hop, outgoing interface, and administrative distance. Static routes are simple but brittle at scale; dynamic protocols (OSPF, IS-IS, BGP) exchange reachability automatically.
Switching (L2) vs routing (L3)
| Aspect | Layer 2 switching | Layer 3 routing |
|---|---|---|
| Forwarding decision | Destination MAC in frame | Destination IP — longest-prefix match |
| Scope | Single broadcast domain per VLAN | Between subnets and across WAN |
| Loops | STP/RSTP blocks redundant switch paths | Routing protocols + TTL prevent infinite IP loops |
| Host sends to… | Direct MAC if same subnet | Default gateway MAC when destination network is remote |
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