Octet

Say this first

It does not advertise a network directly. It matches interface addresses with a wildcard mask and enables OSPF on every interface that matches, in the given area. That interface then sends hellos and its subnet is advertised. The alternative is ip ospf <process> area <area> on the interface, which I prefer.

Go deeper

Network statements

Cisco IOS

router ospf 1
 network 10.0.12.0 0.0.0.3 area 0
 network 10.10.0.0 0.0.255.255 area 1
 network 1.1.1.1 0.0.0.0 area 0

0.0.0.3 with 10.0.12.0 matches 10.0.12.0 to 10.0.12.3.
0.0.0.0 matches one exact interface address. It is the most precise form.
The first matching statement wins, most specific first in the running config.

Interface mode

Cisco IOS

interface GigabitEthernet0/0/0
 ip ospf 1 area 0

It is explicit, cannot match a new interface by accident, and wins over a network statement.

Common mistakes

network 0.0.0.0 255.255.255.255 area 0 enables OSPF on every interface, including the ones to the ISP.
The mask in the network statement is a wildcard, not a subnet mask.
The prefix advertised is the interface's real subnet, not the range in the statement.

They will ask next

Can an interface have secondary addresses in OSPF?
Yes. Secondaries in the same area are advertised as stub networks. Hellos use the primary address.

Traps

Thinking network 10.0.0.0 0.255.255.255 area 0 advertises a /8. It enables matching interfaces and advertises their own subnets.

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