The Application layer protocol defines how application processes like clients and servers running on different end systems, pass messages to each other. The Application layer contains the communications protocol and interface methods used in the process to process communications across an IP (Internet Protocol) computer network. An application layer protocol defines how application processes running on different end systems pass messages to each other. The application layer is present at the top of the OSI (Open system Interconnection ) model. It is the layer through which the user interacts and it provides services to the user. Application layer protocols include TELNET, FTP, NFS, SMTP, LPD, SNMP, DNS etc. The application software that creates the original source data is also important in determining the protocol that is used : the destination application must understand the data being transmitted to it, and for this a well defined communication protocol is needed.