Application Protocol and Services
DNS(Domain Name Services)
- The Domain Name System (DNS) provides a way for hosts to use this name to request the IP address of a specific server.
- DNS names are registered and organized on the Internet within specific high level groups, or domains. Some of the most common high level domains on the Internet are .com, .edu, and .net.
- A DNS server contains a table that associates hostnames in a domain with corresponding IP addresses. When a client has the name of server, such as a web server, but needs to find the IP address, it sends a request to the DNS server on port 53. The client uses the IP address of the DNS server configured in the DNS settings of the host’s IP configuration.
Web Clients and Servers
- When a web client receives the IP address of a web server, the client browser uses that IP address and port 80 to request web services. This request is sent to the server using the Hypertext Transfer Protocol (HTTP).
- When the server receives a port 80 request, the server responds to the client request and sends the web page to the client. The information content of a web page is encoded using specialized ‘mark-up’ languages. HTML (Hypertext Mark-up Language) is the most commonly used but others, such as XML and XHTML, are gaining popularity.
- The HTTP protocol is not a secure protocol; information could easily be intercepted by other users as it is sent over the network. In order to provide security for the data, HTTP can be used with secure transport protocols. Requests for secure HTTP are sent to port 443. These requests require the use of https: in the site address in the browser, rather than http:.
FTP Client and Services
- The File Transfer Protocol (FTP) provides an easy method to transfer files from one computer to another. A host running FTP client software can access an FTP server to perform various file management functions including file uploads and downloads.
- The FTP server enables a client to exchange files between devices. It also enables clients to manage files remotely by sending file management commands such as delete or rename. To accomplish this, the FTP service uses two different ports to communicate between client and server.
- Requests to begin an FTP session are sent to the server using destination port 21. Once the session is opened, the server will change to port 20 to transfer the data files.
- FTP client software is built into computer operating systems and into most web browsers. Stand-alone FTP clients offer many options in an easy-to-use GUI-based interface.
Email Client and Servers
- Email is one of the most popular client/server applications on the Internet. Email servers run server software that enables them to interact with clients and with other email servers over the network.
- Mail servers are also used to send mail addressed to local mailboxes or mailboxes located on other email servers.
- Various application protocols used in processing email include SMTP, POP3, IMAP4.
IM Client and Servers
- Instant Messaging (IM) is one of the most popular communication tools in use today. IM software is run locally on each computer and allows users to communicate or chat over the Internet in real-time
- Each instant messaging service can use a different protocol and destination port, so two hosts must have compatible IM software installed for them to communicate.
- IM applications require minimal configuration to operate. Once the client is downloaded all that is required is to enter username and password information. This allows the IM client to authenticate to the IM network.
- IM client software can be downloaded and used on all types of hosts, including: computers, PDAs and cell phones.
Voice Client and Servers
- IP telephony makes use of Voice over IP (VoIP) technology which uses IP packets to carry digitized voice as data.
- To start using Internet telephone, download the client software from one of the companies that provides the service. Rates for Internet telephone services can vary greatly between regions and providers.
- Calls are made to other users of the same service on the Internet, by selecting the username from a list. A call to a regular telephone (land line or cell phone) requires the use of a gateway to access the Public Switched Telephone Network (PSTN).
DNS, Web, Email, FTP, IM and VoIP are just some of the many services provided by client/server systems over the Internet. These services may be provided by a single server or by several servers.
- Well-Known Ports
Destination ports that are associated with common network applications are identified as well-known ports. These ports are in the range of 1 to 1023.
- Registered Ports
Ports 1024 through 49151 can be used as either source or destination ports. These can be used by organizations to register specific applications such as IM applications.
- Private Ports
Ports 49152 through 65535, often used as source ports. These ports can be used by any application.