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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Datagram Delivery and Routing

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IP Datagram Delivery and Routing
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IP Routing Concepts and the Process of Next-Hop Routing
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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 understand—the division into network ID and host ID bits, as well as the subnet mask—is 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.



Previous Topic/Section
IP Datagram Delivery and Routing
Previous Page
Pages in Current Topic/Section
12
3
Next Page
IP Routing Concepts and the Process of Next-Hop Routing
Next Topic/Section

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