IPv4 addresses are 32 bit addresses divided into 4 octets. In this article we’ll examine how IP addresses are divided into classes and what are they used for. Classful allocation of IP address space wasted many addresses, which exhausted the availability of IPv4 addresses. Today is used  classless addressing system.

Deep understanding of OSPF Link State Advertisments (LSAs) is crucial to understand OSPF behavior. LSAs build up the OSPF link state database (LSDB), and they are exchanged between OSPF enabled nodes. Here you have the list of OSPF LSAs and a short description.

Any routing protocol makes use of timers. They can be hello timers, dead/hold-down timers and so on. On this page you will find default timers values for various routing protocols.

Administrative Distance is reliability assigned by routers to a routing protocol. When a router learns the same route from different routing protocols, then the route learned from protocol with the lowest Administrative Distance (AD) is preferred over the others. For example the router learns a route to 192.168.1.0/25 via OSPF and via RIP, then the one learned via OSPF is the preferred one.