LANs have developed greatly over the last 15 years, there are a large variety of LANs different LANs in different departments and countries equipment can be bought on an incremental basis evolution not revolution - cost savings can be really effective if applications can talk to one another across the different LANs companies with different departments can share resources one solution is for an operating system to provide this multivendor integration alternatively a standard network protocol could be designed

Internet Protocol (IP) - so called because it allows communication between LANs

IP centerpiece of the TCP/IP protocol stack. It hides the differences between data link protocols from the transport protocols that the end user applications use can replace old data link technologies with new 

IP defines a virtual network address space if you are connected to the  Internet then your network has a unique IP address within that network address your machine has a unique host id. IP provides a connectionless packet delivery service it routes small messages  from one machine to another on the addressed within that message  connectionless service routes each packet separately and  therefore does not guarantee reliable delivery having connectionless packet delivery as the basis for all Internet services makes it adaptable to a wide range of hardware connectionless packet delivery is often termed datagram

Transmission Control Protocol

User Data Protocol

Internet Control Message Protocol