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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 Routing Concepts and the Process of Next-Hop Routing 1 2 3 IP Routing In A Subnet Or Classless Addressing (CIDR) Environment

Routing Tables in an Example Internetwork

Let’s consider an example (see Figure 93) with routers R1, R2 and R3 connected in a “triangle”, so that each router can send directly to the others, as well as to its own local network. Suppose R1's local network is 11.0.0.0/8, R2's is 12.0.0.0/8 and R3's is 13.0.0.0/8. (I'm just trying to keep this simple. J) R1 knows that any datagram it sees with 11 as the first octet is on its local network. It will also have a routing entry that says that any IP address starting with “12” should go to R2, and any starting with “13” should go to R3.

Let's suppose that R1 also connects to another router, R4, which has 14.0.0.0/8 as its local network. R1 will have an entry for this local network. However, R2 and R3 also need to know how to reach 14.0.0.0/8, even though they don't connect to it its router directly. Most likely, they will have an entry that says that any datagrams intended for 14.0.0.0/8 should be sent to R1. R1 will then forward them to R4. Similarly, R4 will send any traffic intended for 12.0.0.0/8 or 13.0.0.0/8 through R1.

IP Routing Concepts and the Process of Next-Hop Routing 1 2 3 IP Routing In A Subnet Or Classless Addressing (CIDR) Environment