what is the meaning of
"network 0.0.0.0 0.0.0.0 area n"?
what will be the function of above command?
By default Cisco OSPF will advertise a loopback interface as a /32, no matter what subnet mask you configured on the interface. If you really want the entire subnet advertised then you change the network type of the loopback interface to point to point and then OSPF advertises the entire subnet.
Most of the interface network types are fairly obvious: Ethernet defaults to broadcast, PPP and HDLC default to point to point, frame relay point to point subinterfaces default to point to point. Frame Relay multipoint subinterfaces and Frame Relay on the physical interface default to multipoint. The significance of the network types in OSPF is that OSPF behaves somewhat differently on each type: point to point just forms adjacency and does not bother with DR or BDR. Broadcast must elect DR and BDR and then routers form adjacency with only the DR and BDR. multipoint has a different way of identifying neighbors.
If you have a serial link and the routers on that link have different network types they will probably not form an adjacency.
the command network 0.0.0.0 0.0.0.0 area n does not make much sense. The wildcard mask of 0.0.0.0 specifies a match on all 32 bits which means that only the address 0.0.0.0 could match. But no interface will have address 0.0.0.0.
I wonder if you got this confused with:
network 0.0.0.0 255.255.255.255 area n
which will match all of the interfaces on the router, will put all of these interfaces into the OSPF process and assign them to area n.
It is an easy kind of shortcut way to configure the OSPF to avoid having to type multiple OSPF network statements.