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IP Datagram Direct Delivery and Indirect Delivery (Routing)
(Page 3 of 3)
The Relationship Between Datagram Routing and Addressing
Obviously, each time a datagram must
be sent, it is necessary that we determine first of all whether we can
deliver it directly or if routing is required. Remember all those pages
and pages of details about IP addressing?
Well, this is where the payoff is. The same thing that makes IP addressing
sometimes hard to understandthe
division into network ID and host ID bits,
as well as the subnet
maskis what allows a device to quickly
determine whether or not it is on the same network as its intended recipient:
Key Concept: The delivery of IP datagrams is divided into two categories: direct and indirect. Direct delivery is possible when two devices are on the same physical network. When they are not, indirect delivery, more commonly called routing, is required to get the datagrams from source to destination. A device can tell which type of delivery is required by looking at the IP address of the destination, in conjunction with supplemental information such as the subnet mask that tells the device what network or subnet it is on.
The determination of what type of
delivery is required is the first step in the source deciding where
to send a datagram. If it realizes the destination is on the same local
network it will address the datagram to the recipient directly at the
data link layer. Otherwise, it will send the datagram to the data link
layer address of one of the routers to which it is connected. The IP
address of the datagram will still be that of the ultimate destination.
Mapping between IP addresses and data link layer addresses is accomplished
using the TCP/IP
Address Resolution Protocol (ARP).
I should also clarify one thing regarding
the differentiation between direct and indirect delivery. Routing is
done in the latter case to get the datagram to the local network of
the recipient. After the datagram has been routed to the recipient's
physical network, it is sent to the recipient by the recipient's local
router. So, you could say that indirect delivery includes direct delivery
as its final step.
The next topic discusses IP routing
processes and concepts in more detail.
Note: Strictly speaking, any process of delivery between a source and destination device can be considered routing, even if they are on the same network. It is common, however, for the process of routing to refer more specifically to indirect delivery as explained above.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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