What is the Internet?

ALSO SEE PREZI BELOW

The term 'Internet' comes from the terms, 'Internetwork' & 'Internetworking'. These terms are used to describe a connection between 2 or more networks. Any network of computer networks is an internet or internetwork.

The Internet (definite article, capitalisation) is the 'special' internet that is available to the general public.

The Internet is a wide area network (WAN) - its connections span a geographically wide area.

The World Wide Web

The web began in 1989 in CERN, the idea of Tim Berners-Lee to link together scientific data and allow easy navigation between resources.

Tim Berners-Lee wrote the first web browser in Pascal. The term now refers to the documents that we access via the Internet.

Intranet

An Intranet is what you get when you provide Internet-like services within an organisation. It uses the same protocols as the Internet and the experience of using it is similar. At school, an Intranet is used to publish a number of web pages. It is relatively easy to do this on a home network if you install a web server.

The Networks

The following diagram describes the way that the networks that make up the Internet are connected.

The Internet backbone is made up of many large networks which are connected to one another. These large networks are known as Network Service Providers or NSPs. These networks exchange packet traffic. Each NSP is required to connect to three Network Access Points or NAPs. At the NAPs, packet traffic may jump from one NSP's backbone to another NSP's backbone. NSPs also interconnect at Metropolitan Area Exchanges or MAEs. MAEs are privately owned but perform the same function as the NAPs. NAPs were the original Internet interconnect points. Both NAPs and MAEs are referred to as Internet Exchange Points or IXs. NSPs also sell bandwidth to smaller networks like ISPs.

Packets

Data is sent across networks in manageable chunks called packets. Sometimes packets are known as datagrams. The Internet is based on a technology called packet switching.

Packet (Simple Version)

A packet consists of

• Source address
• Destination address
• Data to transport

Circuit Switching

Circuit switching is used in the public telephone system. When a call is made, a path is set up between the caller and receiver and is maintained for the duration of the call. In a network, a circuit can be established between two devices transmitting data at the same rate. Because data segments arrive in the order that they are sent, less processing time is required to reconstruct messages when they are received.

Search for an animation that demonstrates this - there are many on the WWW.

Packet Switching

Before a message is sent, it is divided up into packets. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

Routers

The information used to get packets to their destinations is contained in routing tables kept by each router connected to the Internet. Routers are packet switches. A router is usually connected between networks to route packets between them. Each router knows about it's sub-networks and which IP addresses they use. The router usually doesn't know what IP addresses are 'above' it.

The black boxes connecting the backbones are routers. The larger NSP backbones at the top are connected at a NAP. Under them are several sub-networks, and under them, more sub-networks.

When a packet arrives at a router, the router checks the destination address that wit was given when it was created. Then the router checks its routing table. If it has the address of the network that contains the destination address, the packet is sent to that network. If it doesn't have the address of the network that contains the IP address in the packet, it sends the packet on a predefined default route, usually up the hierarchy to the next router.

The next router may know the address of the network the packet is heading for. If it does not, it routes the packet up the hierarchy again until either the packet gets sent towards its destination or reaches an NSP.

The routers connected to the NSP backbones hold the largest routing tables. The packet will now be routed towards the correct backbone and will find its way back down the hierarchy to its destination.

The End-To-End Principle

When two computers communicate on the Internet, they are in control of the communication. The role of network is to move packets between the two points. This approach has advantages.

The routers do not have to know if endpoints are connected. The endpoints handle this. This simplifies the design of the router. The role of the Internet is to move packets, the endpoints handle all the details of maintaining security and integrity of information. Endpoints connect to a single router.

Open Architecture

The principles of operation described so far, describe an open architecture network. This term refers to the fact that any network can be connected to the Internet via a router gateway.

Structure of the Internet - IP and DNS.ppt

TCP/IP

Definitions

• Server - a software process that provides a service requested by a client
• Client - a software process that requests and uses services from a server
• Client-server model - a client software process initiates a request for a service from a server software process, and the server responds to that request

Protocols

A protocol, is a set of rules required for successful communication between devices. The protocol defines the signals, codes and rules for transmission of data across the network. Standard protocols conform to standards laid down by a standards authority. This makes communication possible between devices which support the standard.

Network Protocol Stacks

Networking protocols are usually designed in layers, with each layer responsible for a different part of the communication process. The collection of layers that make up the protocol is called the protocol stack.

The Open Systems Interconnect Model

In 1978, the International Standards Organization (ISO) introduced a model for Open Systems Interconnect (OSI) as a first step toward international standardization of the various protocols required for network communication.

The model consists of seven layers, each layer representing a specific set of network functions.

1. Physical Layer

The physical layer is the lowest layer of the OSI model. This layer controls the way unstructured, raw, bit-stream data is sent and received over a physical medium. This layer describes the electrical or optical, mechanical, and functional interfaces to the physical network medium. The physical layer carries the signals for all of the higher layers.

2. Data-link Layer

The data-link layer provides error-free transfer of data frames from one computer to another over the physical layer. It concerns itself with error-checking and data flow control. The layers above this layer can assume virtually error-free transmission over the network.

3. Network Layer

The network layer determines which physical path the data takes, based on the network conditions, the priority of service, and other factors.

4. Transport Layer

The transport layer makes sure that messages are delivered in the order in which they were sent and that there is no loss or duplication.

5. Session Layer

The session layer establishes a communications session between processes running on different computers.

6. Presentation Layer

The presentation layer serves as the data translator for the network. This layer on the sending computer translates data from the format sent by the application layer into a common format. At the receiving computer, the presentation layer translates the common format to a format known to the application layer.

7. Application Layer

The application layer serves as the window for users and application processes to access network services.

TCP/IP

Transport Control Protocol (TCP) and Internet Protocol is used on the Internet. The protocol stack is normally considered to be a four-layer protocol. The layers of the stack are,

• Application
• Transport
• Network (Internet)
• Link

Link Layer

The link layer is responsible for placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network medium. The link layer uses hardware addresses (MAC addresses). It is not the part of the protocol that deals with whether or not packets arrive or how they are sequenced.

Network Layer

The Internet layer is responsible for addressing, packaging, and routing functions.

Transport Layer

TCP is responsible for
the establishment of a TCP connection,
the sequencing and acknowledgment of packets sent,
and the recovery of packets lost during transmission.

TCP Ports

A port is an element added to an address to identify which process to pass data to on the host machine.

Port numbers are allocated to client application connections by the TCP program to allow the server to keep track of the connections.

Sockets

Combining the host IP address and port number gives you the socket. It is an identifier for a particular service on a particular node on a network. There are two kinds, streams (bidirectional) and datagrams.

Client port numbers are allocated incrementally in the range 1024 to 4095.

Servers use universally known port numbers to allow clients to access services more easily. (Port - Server)

• 20 - FTP server, file transfer data
• 21 - FTP server, file transfer control
• 80 & 8080 - Web server
• 25 - SMTP Server
• 100 - POP3 Server
• 23 - Telnet Server

Application Layer

The Application layer provides applications the ability to access the services of the other layers and defines the protocols that applications use to exchange data. There are many Application layer protocols and new protocols are always being developed.

Application Layer Protocols

The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.

The File Transfer Protocol (FTP) is used for interactive file transfer.

The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments.

Telnet, a terminal emulation protocol, is used for logging on remotely to network hosts.

Hypertext Transfer Protocol

A client machine sends a request to a server. The server responds with data (text formatted as HTML, images, audio). TCP establishes the connection for the HTTP to send and receive messages between client and server.

When you request a page from your browser, it,

• Accepts URI from user
• Extracts FQDN and uses DNS server to translate to IP address
• Sends a GET request for the resource specified (port 80 by default)
• Receives the file returned
• Renders the file on screen
• Repeat the process for any other URIs in the document (style/images/audio etc)

File Transfer Protocol

This is the protocol you use to copy files from client to server and server to client. Some FTP servers restrict access. Some require that anonymous be entered as the user name and an email address as the password.

Telnet

Text-based protocol to manage a remote machine. You can send and receive e-mail, read the HTML text of a web page and enter text commands. There is a free client shipped with Windows and many open source options are out there.

POP3 & SMTP

POP3 is for retrieving stored mail. SMTP is for sending the mail

HTTPS

Secure Sockets Layer is a protocol that sits beneath the HTTP application layer. It uses port 443 instead of port 80 and is widely used when security is required. For example,

• Financial transactions
• Logging into company networks remotely
• Personal information

Client Server Model.ppt

Domain Names and IP

Unique Addresses

For the end-to-end principle to work, each machine connected to the Internet must have a unique address to identify it.

The Internet Protocol address or IP address is the mechanism for doing this. We currently are in the last days of IPv4 addresses.

IPv432 bit, 4 byte addresses.

The IP address identifies both the network and the host (the computer on the network).

For example,

192.172.0.1
Network = 192.172.0
Host = 1 

The idea is to make it easier and simpler to route packets to the correct network. The router that is connected to the network can then send the packet to the correct host. IP addresses are written out in the format, 192.172.0.1

There are 4 bytes separated by full stops. Since each portion corresponds to 8 bits, the format is often know as octets. In the early days of the Internet, all addressing required IP addresses.

Domain Names

Whatever you type into your browser's address bar, the resource you request is always retrieved using its IP address. The Domain Name System was created to allow people to access resources on the Internet without having to remember the increasing number of unique IP addresses. It allows us to identify networks and hosts using memorable names.

If I type in www.multiwingspan.co.uk into the address bar of my browser,

• The browser contacts a DNS server to get the IP address.
• A DNS server would start its search for an IP address by contacting one of the root DNS servers. The root servers know the IP addresses for all of the DNS servers that handle the top-level domains (.COM, .NET, .ORG, .UK etc.).
• Your DNS server would ask the root for www.multiwingspan.co.uk, and the root, not knowing the IP address for the site, would return the IP address of the DNS server managing the .UK or .CO.UK domains.
• Your DNS server then sends a query to this server which returns the address of the name servers handling the multiwingspan.co.uk domain.
• It returns the IP address of the server holding the multiwingspan.co.uk pages and delivers the page that you requested.

Redundancy & Caching

There are lots of DNS servers at all the levels of domain. The system is fault-tolerant. It is an example of a distributed database.

DNS servers cache the IP addresses that they are asked to find. The next time it receives a request for a page on that server, it already has the information required to connect to it without contacting the root DNS servers again.

This results in an interesting phenomenon called DNS propagation. When you change the IP address that a domain points to, DNS servers that had previously cached the IP address of your domain will still be using old information. An interesting aspect of the experience of this is that, shortly after making such a change, typing in your domain may result in either of the IP addresses being returned. DNS servers do refresh their caches regularly and the system is relatively efficient. There are some sites with scripts that query DNS servers for you and show you the IP addresses they think your domain should point to. By querying a random selection of DNS servers and seeing the IP addresses returned, you can get a sense of the extent to which the changes that you have made have propagated through the DNS servers.

Top-Level Domains

The top-level domain of www.multiwingspan.co.uk is UK.

There are sveral hundred top-level domains. They include the COM, EDU, GOV, MIL domains as well as unique two-letter combinations for each country.

Second-Level Domains

The second-level domain of www.tbshs.org is TBSHS.

Every name in the top-level domain must be unique. There can be duplication across second-level domains. For example, tbshs.com, tbshs.edu et. could also be used.

Third-Level Domains

The third-level domain for www.multiwingspan.co.uk is MULTIWINGSPAN.

Third-level domains can also be duplicated. www.multiwingspan.org.uk is also permissible, for example.

There are up to 127 levels available although more than around 4 and the address ceases to be memorable and this is less likely to occur.

Host

The leftmost portion of the domain name is the host. This represents the machine on which the resource is located.

WWW refers to a web server.

Registries & Registrars

Internet Registries store the domain names that people have registered along with the IP addresses to which the pages point.

Internet Registrars are private companies that register domains to individuals and organisations.

The relative cost of owning a domain name is relatively low. Hosting companies often register them for you as part of a hosting deal. It is also possible to register a domain and have it point to free web space of the sort you tend to get with your domestic Internet connection. It currently costs around £6 to secure a domain name for 2 years.

Uniform Resource Identifier (URI)

The URI specifies how to access a resource on the Internet.

Uniform Resource Names (URN) identifies a resource by name. urn:isbn 978-0-00-724019-7

Uniform Resource Locators (URL) identify a resource by its network. http://www.highcliffeschool.com

URL

Starts with the protocol eg http: ftp: Then the fully qualified domain name (FQDN). www.microsoft.com. Finally the path on the addressed server /index/index.html