TCP/IP is a layered networking protocol suite. At the bottom layer of the TCP/IP protocol stack is the IP network-layer protocol. IP transmits data in the form of datagrams. IP is connectionless, meaning that datagrams transmitted between source and destination hosts may take different routes across the network. IP is unreliable, in that it provides no guarantee that datagrams will arrive in order or unduplicated, or even arrive at all. If reliability is required, then it must be provided via the use of a reliable higher-layer protocol (e.g., TCP), or within an application.
The original version of IP is IPv4. In the early 1990s, a new version of IP, IPv6, was devised. The most notable difference between IPv4 and IPv6 is that IPv4 uses 32 bits to represent a host address, while IPv6 uses 128 bits, thus allowing for a much larger number of hosts on the world-wide Internet. Currently, IPv4 remains the most widely used version of IP, although in coming years, it is likely to be supplanted by IPv6.
Various transport-layer protocols are layered on top of IP, of which the most widely used are UDP and TCP. UDP is an unreliable datagram protocol. TCP is a reliable, connection-oriented, byte-stream protocol. TCP handles all of the details of connection establishment and termination. TCP also packages data into segments for transmission by IP, and provides sequence numbering for these segments so that they can be acknowledged and assembled in the correct order by the receiver. In addition, TCP provides flow control, to prevent a fast sender from overwhelming a slow receiver, and congestion control, to prevent a fast sender from overwhelming the network.
Refer to the sources of further information listed in Further Information.