Client-Server Model

The web is based on the client-server model. Users run web browsers, to access information on web servers. Web servers "serve up" information and services.

1. The client (user) makes a request for some information or service and sends it to the server.

2. The server cranks away to provide that information or service and returns it the client.

3. The client does some processing of its own, such as formatting the results returned from the server.

Internet Standard Components

1. A client-server model in which the server can collect information from clients, execute scripts, and download applications to run on the client's computer.

2. TCP/IP for all communication between nodes.

3. A common naming scheme for all hosts (Domain Naming System, or DNS), and a method for specifying exactly which host and documents you want to access (Universal Resource Locator, or URL).

4. A common format, Hyper-Text Markup Language, or HTML, to create documents and to create hyperlinks among these documents.

5. A standard protocol, Hyper-Text Transfer Protocol, or HTTP , which defines rules for accessing and transferring information between clients and servers.

• When you type a URL in the Address field of a web browser to access a new site, the name must first be converted into an Internet (IP) address.

• The browser creates a query for the DNS server and submits it on the Internet.

• The query is sent to the nearest DNS server for resolution.

• If the DNS server doesn't know the address, it will send the query to another DNS server further up the hierarchy.

• Once a server is found that can resolve the name, an IP address is sent back to your browser.

• Now the IP address goes into a connection request TCP packet along with the IP address of your computer, so that the destination server doesn't have to do another DNS lookup.

• The packet is sent out over the internet where it hops from router to router until it reaches the destination server.

• At this point, the packets may need to pass through an intermediary, such as a proxy server, a gateway, or a firewall.

  
  

• Filtering is done based on the source IP address and the type of service being requested.

• If the intermediary authorizes the packet, it is sent to the server, which then directs it to an appropriate port.

• Assuming the packets are accepted, the server builds connection response packets to return to the browser.

• Now the browser has link to the server, and the remainder of the URL, if any, is processed.

• After the server completes its transmission, it takes down the connection.

• If the user clicks a hyperlink on the page, a whole new connection is established (although a DNS address lookup may not be required).

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