Imagine Alice puts a letter in the mail that is addressed to Bob, who lives in the town right next to hers. For the letter to reach Bob as quickly as possible, it should be delivered right from Alice’s post office to the post office in Bob’s town, and then to Bob. If the letter is first sent to a post office hundreds of miles away, Alice’s letter could take a lot longer to reach Bob.
Like the postal service, networks are more efficient when messages travel as directly as possible. When a network receives data packets from another network, it will sort and route those packets by subnet so that the packets do not take an inefficient route to their destination.
For a real-world example, suppose an IP packet is addressed to the IP address 192.0.2.15. This IP address is a Class C network, so the network is identified by ”192.0.2” (or to be technically precise, 192.0.2.0/24). Network routers forward the packet to a host on the network indicated by ”192.0.2.”
Once the packet arrives at that network, a router within the network consults its routing table. It does some binary mathematics using its subnet mask of 255.255.255.0, sees the device address ”15” (the rest of the IP address indicates the network), and calculates which subnet the packet should go to. It forwards the packet to the router or switch responsible for delivering packets within that subnet, and the packet arrives at IP address 192.0.2.15 (learn more about routers and switches).
