OSPF and IP

OSPF runs over IP, which means that an OSPF packet transmits with an IP data packet header. The protocol field in the IP header is 89, which identifies it as an OSPF packet and distinguishes it from other packets that use an IP header.

An OSPF route advertisement expresses a destination as an IP address and a variable-length mask. Together, the address and the mask indicate the range of destinations to which the advertisement applies.

Because OSPF can specify a range of networks, it can send one summary advertisement that represents multiple destinations. For example, a summary advertisement for the destination 192.0.2.0 with a mask of 255.255.0.0 describes a single route to destinations 192.0.2.0 to 192.0.2.255.