IP Routing Concepts and the Process of Next-Hop Routing
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The Benefits of Next-Hop Routing
This is a critical concept in how IP works: routing is done on a step-by-step basis, one hop at a time. When we decide to send a datagram to a device on a distant network, we don't know the exact path that the datagram will take; we only have enough information to send it to the correct router to which we are attached. That router, in turn, looks at the IP address of the destination and decides where the datagram should next hop to. This process continues until the datagram reaches the destination host's network, when it is delivered.
Next-hop routing may seem at first like a strange way of communicating datagrams over an internetwork. In fact, it is part of what makes IP so powerful. On each step of the journey to any other host, a router only needs to know where the next step for the datagram is. Without this concept, each device and router would need to know what path to take to every other host on the internet, which would be quite impractical.
As mentioned above, each hop in routing consists of traversal of a physical network. After a source sends a datagram to its local router, the data link layer on the router passes it up to the router's IP layer. There, the datagram's header is examined, and the router decides what the next device is to send the datagram to. It then passes it back down to the data link layer to be sent over one of the router's physical network links, typically to another router. The router will either have a record of the physical addresses of the routers to which it is connected, or it will use ARP to determine these addresses.
Another key concept related to the principle of next-hop routing is that routers are designed to accomplish routing, not hosts. Most hosts are connected using only one router to the rest of the internet (or Internet). It would be a maintenance nightmare to have to give each host the smarts to know how to route to every other host. Instead, hosts only decide if they are sending locally to their own network, or if they are sending to a non-local network. If the latter, they just send the datagram to their router and say here, you take care of this. If a host has a connection to more than one router, it only needs to know which router to use for certain sets of distant networks. How routers decide what to do with the datagrams when they receive them from hosts is the subject of the next topic.
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