Internet: a computer network that interconnects billions of computing devices throughout the world. There are servers store and transmit information, and each devices that are connected to the network are called hosts or end systems.
End system are connected together by a network of communication links and packet switches. When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment, and this resulting package of information are called packets. The receiving end will reassemble the packets to get the original data
A packet switch takes a packet arriving on one of its incoming communication links and forward that packet on one of its outgoing communication links. Two types of switch are routers and link-layer switches.
End systems access the internet through Internet Service Providers (ISPs) like local cable, corporate ISP, university ISP etc. Each ISP is in itself a network of packet switches and communication links; they are managed independently, runs the IP protocol, and conforms to certain naming and address conventions. The Internet is all about connecting end systems to each other, so the ISPs that provide access to end systems must also be interconnected.
To allow the applications on the end systems (distributed applications) talk to each other, each end system attached to the internet need to provide a socket interface that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.
The devices connected to the internet are called End Systems because they are sitting at the end of edges of internet. They are also referred as hosts because they hosts running programs e.g. web server, which can then be further divided into clients and servers.
The network that physically connects an end system to the first router(edge router) on a path from the end system to any other distant end system. I.E in residential access, DSL is used to obtain internet access from local telephone company. They are built on top of telephone line.
Another type of access network is cable internet access. It requires special cable modems, and it typically would connect to the home PC through an Ethernet port.
2 Key Feature:
In a network application, end systems exchange messages with each other. Messages can be in any kind of form. To send a message from a sources to destination, the source breaks long message into smaller chunks of data(packets). Packets will travel through communication links and packet switches.
Most packet switches use store-and-forward transmission at the inputs to the links. This means the packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link. For a switch with rate R bps and L-bit packets, end to end delay is 2L/R.