LANs are smaller networks limited in geographic area. Many LANs connected together allow the Internet to function. But LANs have limitations in scale. Although there have been technological advances to improve the speed of communications, such as Metro Optical, Gigabit, and 10-Gigabit Ethernet, distance is still a problem.

Focusing on the communication between the source and destination computer and intermediate computers at the application layer is one way to get an overview of the Internet architecture. Placing identical instances of an application on all the computers in the network could ease the delivery of messages across the large network. However, this does not scale well. For new software to function properly, it would require new applications installed on every computer in the network. For new hardware to function properly, it would require modifying the software. Any failure of an intermediate computer or the application of the computer would cause a break in the chain of the messages that are passed.

The Internet uses the principle of network layer interconnection. Using the OSI model as an example, the goal is to build the functionality of the network in independent modules. This allows a diversity of LAN technologies at Layers 1 and 2 and a diversity of applications functioning at Layers 5, 6, and 7. The OSI model provides a mechanism where the details of the lower and the upper layers are separated. This allows intermediate networking devices to “relay” traffic without having to bother with the details of the LAN.

This leads to the concept of internetworking, or building networks of networks. A network of networks is called an internet, indicated with the lowercase “i”. When referring to the networks that developed from the DoD on which the Worldwide Web (www) runs, the uppercase “I” is used and is called the Internet. Internetworking must be scalable with regard to the number of networks and computers attached. Internetworking must be able to handle the transport of data across vast distances. It must be flexible to account for constant technological innovations. It must be able to adjust to dynamic conditions on the network. And internetworks must be cost-effective. Internetworks must be designed to permit anytime, anywhere, data communications to anyone.

Figure summarizes the connection of one physical network to another through a special purpose computer called a router. These networks are described as directly connected to the router. The router is needed to handle any path decisions required for the two networks to communicate. Many routers are needed to handle large volumes of network traffic.

Figure extends the idea to three physical networks connected by two routers. Routers make complex decisions to allow all the users on all the networks to communicate with each other. Not all networks are directly connected to one another. The router must have some method to handle this situation. 

One option is for a router to keep a list of all computers and all the paths to them. The router would then decide how to forward data packets based on this reference table. The forwarding is based on the IP address of the destination computer. This option would become difficult as the number of users grows. Scalability is introduced when the router keeps a list of all networks, but leaves the local delivery details to the local physical networks. In this situation, the routers pass messages to other routers. Each router shares information about which networks it is connected to. This builds the routing table.

Figure shows the transparency that users require. Yet, the physical and logical structures inside the Internet cloud can be extremely complex as displayed in Figure . The Internet has grown rapidly to allow more and more users. The fact that the Internet has grown so large with more than 90,000 core routes and 300,000,000 end users is proof of the soundness of the Internet architecture.

Two computers, anywhere in the world, following certain hardware, software, and protocol specifications, can communicate reliably. Standardization of practices and procedures for moving data across networks has made the Internet possible.

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