OSPF classifies networks into two four types, depending on the link layer protocols:
Broadcast
OSPF defaults networks using Ethernet or Fiber Distributed Data Interface (FDDI) at the link layer to broadcast networks.
On broadcast networks:
Hello packets, LSU packets, and LSAck packets are sent in multicast mode. The address 224.0.0.5 is the IP multicast address reserved for OSPF DR others. The address 224.0.0.6 is the IP multicast address reserved for OSPF DRs or BDRs.
DD and LSR packets are sent in unicast mode.
Non-Broadcast Multi-Access (NBMA)
OSPF defaults networks using frame relay (FR) or X.25 at the link layer to NBMA networks.
On NBMA networks, protocol packets such as Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are all sent in unicast mode.
Point-to-Multipoint (P2MP)
OSPF does not default a network to a P2MP network regardless of the link layer protocol used by the network. A P2MP network must be changed from another network type. It is a typical practice to change partial-meshed NBMA networks to P2MP networks.
On P2MP networks:
Hello packets are transmitted in multicast mode using the multicast address 224.0.0.5.
DD packets, LSR packets, LSU packets, and LSAck packets are sent in unicast mode.
Point-to-Point (P2P)
OSPF defaults networks using Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), or Link Access Procedure Balanced (LAPB) at the link layer to P2P networks.
On P2P networks, protocol packets such as Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are all sent in multicast mode using the multicast address 224.0.0.5.