OSPF area
OSPF divides a large network into "areas" to limit how far routing information (LSAs) spreads — otherwise every router would need to know the whole network's topology. All areas must connect to the backbone area (area 0).
Mechanic: within an area all routers have an identical topology database and flood LSAs freely; between areas the information is summarized at Area Border Routers (ABR) and crosses via the backbone (area 0). This limits LSA flooding and database size → OSPF scales. Area types: standard, stub (doesn't let in external routes, just a default), totally stubby (even more limited), and NSSA (a stub that can still take in some external routes). Design rule: everything must hang together via area 0 (non-backbone areas can't be islands). Win: faster convergence, less memory/CPU, less flooding in large networks. Related to OSPF LSA and OSPF DR/BDR.