The fourth layer of OSI model is the transport layer.Transport layer is responsible for end to end communication, that is the source and target systems in the network communication between the co-ordination.In the IP protocol stack is TCP (a transport protocol) and UDP (User Datagram Protocol) protocol layer is located.
In the fourth layer, TCP and UDP header contains the port number (portnumber), they can be the only distinction which each packet contains the application protocol (eg HTTP, FTP, etc.).Endpoint system uses this information to distinguish the data packets, in particular, is the port number to a receiver computer system to determine the IP packet type received by it and give it to the appropriate high-level software.IP address port number and device combination is often called "jack (socket)".1 and the port number 255 is reserved, they called the "well known" ports, that is, all host TCP / IP protocol stack implementation, the port number is the same.In addition to "known" ports, standard UNIX services allocated in the 256-1024 port range, custom applications are generally more than the allocated port number 1024.The most recent list of port numbers assigned in the RFc1700 "Assigned Numbers" found.TCP / UDP port number to provide additional information that can be used by the network switch, which is the basis of layer 4 switching.
"Known" port number example:
Application protocol port number
FTP 20 (data) 21 (control)
TELNET 23
SMTP 25
HTTP 80
NNTP 119
NNMP 16
162 (SNMP traps)
TCP / UDP port number to provide additional information that can be used by the network switch, which is the basis for exchange of the fourth floor.
The fourth floor features a switch connected to the server can play a "virtual IP" (VIP) front-end role.
Each server and support a single or universal application server group are configured to a VIP address.The VIP address is sent out and in the domain name registered on the system.