How does a router determine whether datagram to a particular host can be directly delivered through one of its interfaces?

Question

What will be an ideal response?

Answer 1

A router should have the routing table and it will search its contents to find the network route
corresponding to its interfaces by determining its reachability using longest matching method. If
the datagram is for the destination of the same network of the router, it means that the datagram is
for the particular directly connected host.
Usual searching sequence is Hosts which have the longest 32bit mask, subnets depending on its
number of the subnet mask bits of which longer one is preferred, and default gateway.
And for this purpose, router should maintain its routing table. The router chooses which route is
the best among those presented by routing protocols, manual configuration, and various other
means. There are three processes involved in building and maintaining the routing table in a Cisco
router:
? Various routing processes, which actually run a network (or routing) protocol, such as
EIGRP, BGP, IS-IS, and OSPF.
? The routing table itself, which accepts information from the routing processes and
also replies to requests for information from the forwarding process.
? The forwarding process, which requests information from the routing table to make a
packet forwarding decision.

Therefore, given well-maintained routing table, the forwarding process determines whether a particular host is reachable. If a packet arrives on a router interface, choosing proper route by router depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet. Classful and classless routing has a little difference. In classful routing, the router won't forward packets to supernets. If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped. Inclassless routing, the router ignores the classful boundaries of the networks in its routing table and simply route to the longest prefix match it can find.