cisco 6

The term server refers to a host running a software application that provides information or services to other hosts connected to the network. A client is the name given to a computer application that someone uses to access information held on a server. A web browser is a good example of a client.
The key characteristic of client/server systems is that the client sends a request to a server, and the server responds by carrying out a function, such as sending information back to the client. The combination of a web browser and a web server is perhaps the most commonly used instance of a client/server system.
Application Protocol
Hypertext Transfer Protocol (HTTP) governs the way that a web server and a web client interact. HTTP defines the format of the requests and responses exchanged between the client and server. HTTP relies on other protocols to govern how the messages are transported between client and server.
Transport Protocol
Transmission Control Protocol (TCP) is the transport protocol that manages the individual conversations between web servers and web clients. TCP formats the HTTP messages into segments to be sent to the destination host. It also provides flow control and acknowledgement of packets exchanged between hosts
Internetwork Protocol
The most common internetwork protocol is Internet Protocol (IP). IP is responsible for taking the formatted segments from TCP, assigning the logical addressing, and encapsulating them into packets for routing to the destination host.
Ethernet is the most commonly used protocol for local networks. Network access protocols perform two primary functions, data link management and physical network transmissions.Data link management protocols take the packets from IP and encapsulate them into the appropriate frame format for the local network. These protocols assign the physical addresses to the frames and prepare them to be transmitted over the network.
The standards and protocols for the physical media govern how the bits are represented on the media, how the signals are sent over the media, and how they are interpreted by the receiving hosts. Network interface cards implement the appropriate protocols for the media that is being used.
UDP is a 'best effort' delivery system that does not require acknowledgment of receipt. This is similar to sending a standard letter through the postal system. It is not guaranteed that the letter is received, but the chances are good.
UDP is preferable with applications such as streaming audio, video and voice over IP (VoIP). Acknowledgments would slow down delivery and retransmissions are undesirable.
Destination Port
The client places a destination port number in the segment to tell the destination server what service is being requested. For example, Port 80 refers to HTTP or web service. When a client specifies Port 80 in the destination port, the server that receives the message knows that web services are being requested. A server can offer more than one service simultaneously. For example, a server can offer web services on Port 80 at the same time that it offers FTP connection establishment on Port 21.
Source Port
The source port number is randomly generated by the sending device to identify a conversation between two devices. This allows multiple conversations to occur simultaneously. In other words, multiple devices can request HTTP service from a web server at the same time. The separate conversations are tracked based on the source ports.
The source and destination ports are placed within the segment. The segments are then encapsulated within an IP packet. The IP packet contains the IP address of the source and destination. The combination of the source and destination IP address and the source and destination port number is known as a socket. The socket is used to identify the server and service being requested by the client. Every day thousands of hosts communicate with thousands of different servers. Those communications are identified by the sockets.
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.
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.
Each mail server receives and stores mail for users who have mailboxes configured on the mail server. Each user with a mailbox must then use an email client to access the mail server and read these messages. Various application protocols used in processing email include SMTP, POP3, IMAP4.
he following are typical entries when configuring an email client:
POP3 or IMAP4 Server nameSMTP Server nameUsername User passwordSPAM and Virus filters
IM client software can be downloaded and used on all types of hosts, including: computers, PDAs and cell phones.
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).
The protocols and destination ports used by Internet telephony applications can vary based on the software.
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.
The table shows some of the more common well-known ports.
The layered model has many benefits:
Assists in protocol design, because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below. Fosters competition because products from different vendors can work together. Prevents technology or capability changes in one layer from affecting other layers above and below. Provides a common language to describe networking functions and capabilities.
Next, the IP packet is sent to the Ethernet protocol where it is encapsulated in a frame header and trailer. Each Ethernet frame header contains a source and destination MAC address. The trailer contains error checking information. Finally the bits are encoded onto the Ethernet media (copper or fiber optic cable) by the server NIC.