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* IP layer in end-system
hosts and in the router work together to route packets from source to
destination.
* IP layer in each host
and router maintains a routing table, which is used to route the packets based
on IP address.
* If a destination host is
directly connected to the originating host by a link or by a LAN, then the
packet is sent directly to destination host using appropriate network interface,
otherwise, the routing table specifies that the packet is to send to default gateway.
* When a router receives
an IP packet from one of the network interfaces, then router examines its
routing table to see whether the packet is destined to itself or not, if so, delivers
to router’s own address, then the router determines the next–hop router and associated
network interface, and then forwards the packet.
* Each row in routing
table must provide information like: destination IP address, IP address of
next-hop router, several flag fields, outgoing network interface, and other information
such as subnet mask, physical address.
* H flag indicates whether
the route in the given row is to a host (H=1) or to a
network.
* G flag indicates whether
the route in the given row is to a router (gateway, G=1) or to a directly
connected destination (G=0).
* Each time a packet is to
be routed, the routing table is searched in the following order.
* First, the destination
column is searched to see whether table contains an entry for complete
destination IP address.
* If so, then IP packet is
forwarded according to next-hop entry and G flag.
* Second, if the table
does not contain complete destination IP address, then routing table is
searched for the destination network ID.
* If an entry found, the
IP packet is forwarded according to next-hop entry and G flag.
* Third, if table does not
contain destination network ID, the table is searched for default router entry,
and if one is available, the packet is forwarded there.
* Finally if none of the
above searches are successful, the packet is declared
undeliverable and an ICMP
“host unreachable error” packet is sent back to
originating host.
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