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

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:

• Conventional “Classful” Addressing: We know the class of each address by looking at the first few bits. This tells us which bits of an address are the network ID. If the network ID of the destination is the same as our own, the recipient is on the same network; otherwise, it is not.
  
  
• Subnetted “Classful” Addressing: We use our subnet mask to determine our network ID and subnet ID and that of the destination address. If the network ID and subnet are the same, the recipient is on the same subnet. If only the network ID is the same, the recipient is on a different subnet of the same network. If the network ID is different, the destination is on a different network entirely.
  
  
• Classless Addressing: The same basic technique is used as for subnetted “classful” addressing, except that there are no subnets. We use the “slash number” to determine what part of the address is the network ID and compare the source and destination as before. There are complications here, however, that I discuss more in the topic on routing in a classless environment.

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.

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