The first week of CST 311 provided an overview of computer networking, and the topology that makes up the Internet.
Two important parts of the Internet are the network edge, and the network core. The network edge consists of end-systems, also called hosts, (because they “host” applications). End systems are what you typically think of internet-connected devices — computers, servers, smart TVs, etc. The network edge also contains the infrastructure that make up access networks, such as switches and routers. Note that “access networks” are essentially end-user networks that connect to the Internet, such as residential Internet connections. Access networks usually pay an ISP for their connection to the Internet. In contrast, the network core is the infrastructure that ISPs use to connect access networks. The network core is the glue that creates a “network of networks” — it is what physically links all devices that are connected to the Internet.
The internet itself, like any communication medium, needs a series of protocols that define how communication occurs. Five protocol “layers” are used to implement the Internet – the “Application” layer, the “Transport” layer, the “Network” layer, the “Link” layer, and the “Physical” layer. Each layer encapsulates a primary objective, and provides that service to the layer above it. Broadly speaking, each layer does the following:
- Application: Facilitates network applications and their protocols. Some common application-layer protocols include HTTP, SMTP, FTP, and DNS.
- Transport: Responsible for transporting application-layer packets from the source endpoint to a specific port at the destination endpoint. There are two transport-layer protocols: TCP and UDP.
- Network: Implements the movement of transport-layer packets between hosts. The widely known “Internet Protocol” is a part of the network layer.
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Link: Moves network-layer packets between individual nodes
of the network.
- Physical: Handles the transmission and propagation of bits between nodes. The protocols of this layer depend on the physical medium of the link.