 in the network layer. Though the network layer protocol is usually connectionless, it can also be con- nection oriented. That is, the network layer protocol supports both connectionless and connection-oriented protocols. When connectionless protocol like IP is used in the network layer, data consistency and reliability are handled using connection- oriented protocols like TCP in the transport layer. IP (Internet protocol) is the most popular network layer protocol. There are sev- eral other supporting protocols of IP for security, traffic control, error handling, and SO on that reside in the network layer. These protocols include IPsec, IP NAT, and ICMP. Another network layer protocol outside the realm of the Internet protocol is IPX (inter-network packet exchange) and DDP (datagram delivery protocol). 10.5 Transport Layer This is the fourth lowest layer (L4) in the ISO/OSI model. While the physical layer is concerned with transmission of bits, the data link layer with transmission in local area, and the network layer with transition even with remote systems, the trans- port layer handles transmission of data between processes/applications. There will be multiple applications running on the same system and there would be scenarios when more than one application from a specific source machine wants to send data to multiple applications on a destination machine. Transport layer ensures that mul- tiple applications on the same system can communicate to another set of applica- tions on a different system. This would mean that a new parameter in addition to the destination and source IP address is required for application level communication. So a transport protocol can have simultaneous connections to a computer and ensure that the receiving computer still knows what application should handle each data packet that is received. This task is accomplished using "ports." The parameter port is just an internal address that facilitates control of data flow. Ports that have specific purposes and used for generic applicatio