Open Shortest Path First (OSPF) is an interior gateway, link state routing protocol. OSPF routers exchange information about the state of the links in the network, and this information are entered into the topology database. The Shortest Path First (SPF) algorithm is then run on the contents of the topology or link state database to build the routing table.
Basic OSPF configuration involves enabling an OSPF routing process on a router and specifying a range of IP addresses associated with the routing process along with the corresponding area ID.
There are two ways to enable the OSPF process on an interface
1) Using legacy Network statement under OSPF process
2) Using Interface level command ip ospf [Process-id] area [area-id]
Both enable OSPF process on an interface except, if an interface is unnumbered and there is a network statement match the IP address of the primary interface, both the primary interface and the unnumbered interface will have OSPF enable on them in assigned area. If multiple network statements configured for same interface then most specific match base on the wildcard mask wins.