How web servers work

The process of accessing a web page

I typed a URL(http://www.xxx.com/index.html) into  my browser and press enter.

The browser broke the URL into three parts: 

The protocol("http")

The server name ("www.xxx.com")

The file name ("index.html")

The browser communicated with a name server to translate the server name "www.xxx.com" into IP Address, which it uses to connect to the server machine. The browser then formed a connection to that server at that IP Address on port 80.

Following the HTTP protocol, the browser sent a GET request to the server, asking for the file ("index.html").

The server then sent the html text for the web page to the browser. The browser read the html tags and formatted the page onto my screen.

The Internet

It's a collection of millions of computers, all linked together on a computer network. The network allows all of the computers to communicate with one another. 


Clients and Servers

Those machines that provide services to other machines are servers. And the machines that are used to connect to those services are clients.

IP Address

To keep all of these machines straight, each machine on the Internet is assigned a unique address called an IP
address. IP stands for Internet
protocol, and these addresses are 32-bit
numbers, normally expressed as four "octets" in a "dotted decimal number." 

Domain Names

Because most people have trouble remembering the strings of numbers that make up IP Addresses, and because IP addresses sometimes
need to change, all servers on the internet also have human-readable names, called domain names. The name https://www.google.com is a permanent, human-readable name. It is easier for most of us to remember. The name https://www.google.com actually has three
parts:

1. The host name ("www")

2. The domain name ("google")

3. The top-level domain name ("com")

Name Servers

A set of servers called domain name servers(DNS) maps the human-readable names to the IP addresses. These servers are simple databases
that map names to IP addresses. and they are distributed all over the internet. Most individual companies, ISPs and universities maintain small name servers to map host names to IP addresses.

Ports

Any server machine makes its services available to the internet using numbered ports, one for each service that is available on the server. 

For example, if a server machine is running a Web server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. Clients connect to a service at a specific IP
address and on a specific port.

Each of the most well-known services is available at a well-known port number. Here are some common port numbers:

echo 7
daytime 13
qotd 17 (Quote of the Day)
ftp 21
telnet 23
smtp 25 (Simple Mail Transfer, meaning e-mail)
time 37
nameserver 53
nicname 43 (Who Is)
gopher 70
finger 79
WWW 80

Protocol

Once a client has connected to a service on a particular port, it accesses the service using a specific protocol. The protocol is
the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have
their conversation.