Null interfaces are virtual interfaces and are always up. A virtual interface is not a physical interface like Fast Ethernet interface or Gigabit Ethernet interface.
Null interfaces never forward or receive traffic; packets routed to a null interface are dropped. Null interface is a logical interface absorbs packets without forwarding them to another interface or destination. Null interface is also known as bit bucket because the IP datagram reaching Null interface are dropped as soon as they are received.
The Null interface in a Cisco Router is a mechanism for preventing routing loops. Enhanced Interior Gateway Routing Protocol (EIGRP) creates a route to the Null0 interface when it summarizes a group of routes.