OSPF uses the concept of "area" to describe different chunks of the network. The center of the network is "area 0", the backbone area, and it's assumed that all areas connect to area 0.
Router interfaces, not entire routers are part of an area. So a router can span more than one area -- for example, a router could have one interface in area 0 and another interface in area 1 and a third in area 199. Other routers in area 1 would then see the router as a path back to area 0.
Areas can be defined as "stubby", meaning that there's only one path out from the area, and so external routes don't get advertised to the stubby area. A "not so stubby area" (nssa) can receive intra-area routes, but no external routes.
- ▼ October (9)