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)

IP Message Reassembly Process 1 IP Datagram Direct Delivery and Indirect Delivery (Routing)

The essential functions of IP datagram encapsulation and addressing are sometimes compared to putting a letter in an envelope and then writing the address of the recipient on it. Once our IP datagram “envelope” is filled and labelled, it is ready to go, but it's still sitting on our desk. The last of the main functions of IP is to get the envelope from us to our intended recipient. This is the process of datagram delivery. When the recipient is not on our local network, this delivery requires that the datagram be routed from our network to the one where the destination resides.

In this section I discuss some of the particulars of how IP routes datagrams over an internetwork. I begin with an overview of the process and contrast direct and indirect delivery of data between devices. I discuss the main method used to route datagrams over the internet, and also explain briefly how IP routing tables are built and maintained. I discuss how the move from “classful” to classless addressing using CIDR impacted routing.

IP Message Reassembly Process 1 IP Datagram Direct Delivery and Indirect Delivery (Routing)