Introduction to the Internet

mb.sb/net

This page is intended as teaching material for introducing computer networking and the Internet.
To teach yourself networking, check out these excellent resources, some of the content here is based on them:
- "Practical Networking" by Ed Harmoush
- "Computer Networking: A Top-Down Approach" by Jim Kurose and Keith Ross
Globe shows submarine Internet cables schematically. Video source.

Hosts

Hosts are computers at the beginning or end of network traffic:

Client and Server

The client initiates the communication. First, the client sends a request. Then, the server responds.
Server: A computer with software that can respond to requests.

The client-server style of communication between two computers is very common. You use it every time when downloading a web page.
Being a client or being a server is the role a host plays.
Mind that the roles of client and servers are not fixed: The web server at www.example.com might request software updates from another server. In this case, the computer at www.example.com becomes the client of that update server because it asks the update server for updates, it initiates the communication.
Client and server use the Hypertext Transfer Protocol (HTTP):
- The client uses HTTP to request a web page.
- The server uses HTTP to respond with the web page.

What are IP addresses?

IP means "Internet Protocol"
An IP address is a number that identifies a host.
We need a host's IP address to send data to it over the network.

Client to server communication with source an destination IP addresses shown

Requests and responses are put into IP packets. The IP packet contains the source and destination IP addresses. After that comes the payload of the packet: A request or response in our case.

IP addresses are hierarchical

Move to show IP addresses:

The example company has the IP address space starting with 10. The company can assign one of its branches a subnet, for example 10.20.x.x.

This subnet can again be divided up into different subnets like 10.20.55.x, forming a hierarchy.

A host in that subnet is for example 10.20.55.8. The hierarchy of the network allows us to find this host using its IP address.

Networks

A network transports data between hosts.
Networks connect to other networks.
The Internet is a network of networks.

Move to show networks:

The core purpose of networking is to connect hosts with each other.
The simplest network consists of two connected hosts.
Before networking, you had to exchange data by putting the data on a portable medium (e.g. diskettes) and carry it over to the target host.
Instead of connecting all networks individually, they are connected by internet service providers (ISPs). All those networks form the Internet.

Create a single network with a switch

Move to build network:

A switch connects hosts directly, creating a single network. The switch and the hosts form a local area network (LAN).
The switch forwards packets from one host to the destination host within the same network.
To forward traffic from one network to the next, we need a router.

How switches work.

Connect networks with a router

Move to connect two networks:

Routers connect the hierarchy of networks

Move to connect all networks:

Routers connect networks, forwarding data between them.
Routers sit between networks and have one IP address for each network.
Simplifying a bit, the Internet is just networks connected with routers.

Summary: Switches, routers, and hosts

Hosts with switches and router in between

Creates a network: Switch
Connects a network to another network: Router
Uses networks: Host (laptop, server, phone)

Switches and routers connect hosts. They are computers that forward traffic to and from hosts.
Hosts are at the beginning and end of traffic, switches and routers are between hosts.

Combined device: Residential gateway

A residential gateway is commonly called just a "router" but it's also a switch, wireless access point (for Wi-Fi), and often provides a local web server for configuring the network.

The Internet

Home network → Internet

Move to show path through Internet:

A (somewhat hierarchical) network of networks

Home and enterprise networks connect to regional ISPs (internet service providers) which again connect to global ISPs.
Global ISPs are connected to each other.
Content providers like Google and Amazon have their own networks connected to their data centers. These data centers are distributed world wide so data from content providers doesn't have to be transmitted far. A content provider's networks directly connect to regional ISPs to partially bypass global ISPs. This allows content providers to negotiate smaller fees when connecting to global ISPs.