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The Internet

The Internet

The ICANN accredits domain name registrars who register domain names for clients so they stay unique. All network applications accept a host address given either as a domain name or an IP address. In fact, a domain name is first translated to a numerical IP address before being used.

Packet Switching
Data on the Internet are sent and received in packets. A packet envelops transmitted data with address information so the data can be routed through intermediate computers on the network. Because there are multiple routes from the source to the destination host, the Internet is very reliable and can operate even if parts of the network are down.

Client and Server
Most commonly, a network application involves a server and a client. A server process provides a specific service on a host machine that offers such a service.

Example services are remote host access (TELNET), file transfer (FTP), and the World- Wide Web (HTTP). Each Internet standard service has its own unique port number that are identical across all hosts. The port number together with the Internet address of a host identifies a particular server anywhere on the Network. For example, FTP has port number 21, Telnet 23, and HTTP 80.

A client process on a host connects with a server on another host to obtain its service. Thus, a client program is the agent through which a particular network service can be obtained. Different agents are usually required for different services.

A Web browser such as Netscape is an HTTP client. It runs on your computer to access Web servers on any Internet hosts.

The Domain Name System
As stated in Section 1.2, every host on the Internet has a unique IP address and a domain name. The network name space is the set of all host names and changes dynamically with time due to addition/deletion of hosts, regrouping of local work groups, reconfiguration of subparts of the network, maintenance of systems and networks, and so on. So, new do- main names, new IP addresses, and new domain-to-IP associations can be introduced in the name space at any time without central control. The domain name system (DNS) provides a distributed database service that supports dynamic update and retrieval of information contained in the name space . A network client program (e.g. the Netscape Nav- igator browser) will normally use the DNS to obtain address information for a target host before making contact with a server. The dynamic DNS also supplies a general mechanism for retrieving many kinds of information about hosts and even individual users.

Here are points to note about the DNS name space: • The DNS organizes the entire Internet name space into a big tree structure. Each nodeof the tree has a label and a list of resources.

• Labels are character strings (currently not case-sensitive) and sibling labels must be distinct. The root is labeled by the empty string. Immediately below the root are the top-level domains: edu, com, gov, net, org, and so on. TLDs also include country names–at (Austria), ca (Canada), cn (China), for example. Under edu, for example, there are subdomains berkeley, kent, mit, uiuc, and so on.
• A full domain name of a node is a dot-separated list of labels leading from the node up to the root (cs.kent.edu., for example).
• A relative domain name is a prefix of a full domain name indicating a node relative to a domain of origin. Thus, the familiar cs.kent.edu is actually a name relative to the root.
• A label is the formal or canonical name of a domain. Alternative names, called aliases, are also allowed. For example, the main Web server host info has the alias www so it is also known as www.cs.kent.edu. To move the Web server to a different host, a local system manager simply reassigns the alias to another host.

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