6.1L2/L3 fundamentals (MAC, VLAN, IP, subnet, gateway)

For someone writing automation, network fundamentals are not a memorization subject but the foundation for judging whether packets actually reach the API endpoint your script calls. When requests.get() times out, an ssh session will not open, or a NETCONF session fails to establish, the cause is often not an application bug but L2/L3 reachability. This section organizes MAC/VLAN (L2), which handle forwarding within one segment, and IP/subnet/routing/default gateway (L3), which handle reachability across segments, as tools for isolating "where it stops."

6.1.1L2: MAC addresses and VLANs

• A MAC address is a 48-bit L2 identifier burned into the NIC (e.g. 00:1A:2B:3C:4D:5E). Within one segment (same VLAN/subnet), a switch forwards a frame to the destination MAC. To a different subnet it cannot reach by MAC; L3 routing is required—confusing this leads to misdiagnosing "they are on the same switch yet cannot communicate."
• A VLAN logically splits one physical switch into multiple broadcast domains. Different VLANs are separate segments at L2, so inter-VLAN traffic requires L3 (a router or an L3 switch SVI). If the automation host sits in a different VLAN from the managed devices but routing or a default gateway is not set, it cannot reach them even within the same building.