Which IP multicast address does OSPF use for communication with all routers?

OSPF (Open Shortest Path First) utilizes the multicast address 224.0.0.5 for sending routing updates to all OSPF routers on a local network segment. This address is designated specifically for all OSPF routers and is part of the reserved range of multicast addresses designated for network protocol communications.

When an OSPF router sends packets to 224.0.0.5, all OSPF-capable routers in the same subnet receive these packets, enabling efficient distribution of routing information without the need for sending individual unicast messages to each router. This multicast mechanism optimizes bandwidth usage and simplifies the process of routing updates across the network.

The other options represent different multicast addresses, but they serve other purposes or protocols. For example, 239.255.255.255 is used for administratively scoped multicast, and 224.0.0.6 is specifically used by OSPF for communication with designated routers, not all routers. Thus, while these addresses are relevant in the context of multicast networking, they are not the address utilized by OSPF for communicating with all routers.