Internet Protocol (IP), introduced in 1974 as part of a larger suite, is a communication protocol defined to provide the transmission of datagrams across network boundaries. As it sits at layer 3, it is above LAN and WAN technologies that include Ethernet, ATM, and Frame Relay. It uses IP addressing and netmasks to uniquely identify every device in the network, regardless of the access technology used to connect them. IP is part of a larger protocol suite that initially included TCP for connection oriented reliable transmission of packets and later added UDP for connectionless transport.
An IPv4 packet consists of a header, of at least 20 bytes in length, and payload, with the header indicating the source and destination address, the payload type, and an indication as to the Type of Service expected. In modern networks, the IP packet is encapsulated within an Ethernet frame for transmission and is therefore limited in size from 46 bytes to 1500 bytes. A standard Ethernet header adds another 18 bytes giving a frame size of 64 bytes to 1518 bytes.
IPv4 routing uses masks to subdivide the IP address space, represented as 4 octets (i.e. 18.104.22.168) into smaller networks. The full IP address space is broken down into 3 main classifications, Class A, B, and C, with 1 byte, 2 byte, 3 byte masks respectively. Class D and E further define reserved address ranges for multicast and future applications. This segmentation was later deemed to course and was replaced with a variable length subnet mask (VLSM) and Classless Interdomain Routing (CIDR). These new address blocks are managed and allocated by Internet Address Numbers Authority and Regional Internet Registry.
Devices assigned a unique IP address and accessible publicly can be considered part of the larger Internet.