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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

IP Datagram Delivery and Routing 1 2 3 IP Routing Concepts and the Process of Next-Hop Routing

Comparing Direct and Indirect Delivery

Direct delivery is obviously the simpler of these. The source just sends the IP datagram down to its data link layer implementation. The data link layer encapsulates the datagram in a frame that is sent over the physical network directly to the recipient's data link layer, which passes it up to the IP layer.

Indirect delivery is much more complicated, because we can't send the data straight to the recipient. In fact, we usually will not even know where the recipient is, exactly. Sure, we have its address, but we may not know what network it is on, or where that network is relative to our own. (If I told you my address you'd know it's somewhere in Bennington, Vermont, but could you find it?) Like relying on the postal system in the envelope analogy, we must rely on the internetwork itself to indirectly deliver datagrams. And like the postal system, the power of IP is that you don't need to know how to get the letter to its recipient; you just put it into the system.

The devices that accomplish this “magic” of indirect delivery are generally known as routers, and indirect delivery is more commonly called routing. Like entrusting a letter to your local mail carrier or mailbox, a host that needs to send to a distant device generally sends datagrams to its local router. The router connects to one or more other routers, and they each maintain information about where to send datagrams so that they reach their final destination.

Indirect delivery is almost always required when communicating with distant devices, such as those on the Internet or across a WAN link. However, it may also be needed even to send to a device in the next room of your office, if that device is not connected directly to yours at layer two.

IP Datagram Delivery and Routing 1 2 3 IP Routing Concepts and the Process of Next-Hop Routing