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The Internet Protocol ('''IP''') is a data-oriented Protocol used for communicating data across a Packet-switched Internetwork . IP is a Network Layer protocol in the Internet Protocol Suite and is Encapsulated in a Data Link Layer protocol (e.g., Ethernet ). As a Lower Layer Protocol , IP provides the service of ''communicable'' unique global addressing amongst computers. This implies that the data link layer need not provide this service. Ethernet provides globally unique addresses except it is not globally communicable (i.e., two arbitrarily chosen ethernet devices will only be able to communicate if they are on the same bus). PACKETIZATION datagram inside an IP packet.]] Data from an Upper Layer Protocol is encapsulated inside one or more Packet s/ Datagram s (the terms are basically synonymous in IP). No Circuit setup is needed before a host tries to send packets to a host it has previously not communicated with (this is the point of a Packet-switched network). This is quite unlike Public Switched Telephone Network s that require the setup of a circuit before a phone call may go through. SERVICES PROVIDED BY IP Because of the abstraction provided by encapsulation, IP can be used over a Heterogenous network (i.e., a network connecting two computers can be any mix of Ethernet , ATM , FDDI , Wi-fi , Token Ring , etc.) and it makes no difference to the Upper Layer Protocol s. All the data link layers can (and do) have their own set of addressing (or possibly the complete lack of it) and the need to resolve IP addresses to data link addresses is needed. This resolving is addressed by the Address Resolution Protocol (ARP). RELIABILITY IP provides an ''unreliable'' service (i.e., Best Effort Delivery ). This means that the network makes no guarantees about the packet and none, some, or all of the following may apply:
In terms of reliability the only thing IP does is ensure the IP packet's header is error-free through the use of a Checksum . This has the side-effect of discarding packets with bad headers on the spot, and with no required notification to either end (though an ICMP message may be sent). To address any of these reliability issues, an Upper Layer Protocol must handle it. For example, to ensure in-order delivery the upper layer may have to cache data until it can be passed up in order. The primary reason for the lack of reliability is to reduce the complexity of Router s. While this does give routers '' Carte Blanche '' to do as they please with packets, anything less than ''best effort'' yields a poorer experience for the user. So, even though no guarantees are made, the better the effort made by the network, the better the experience for the user. IP ADDRESSING AND ROUTING Perhaps the most complex aspects of IP are addressing and Routing . Addressing refers to how end hosts become assigned IP Address es and how subnetworks of IP host addresses are divided and grouped together. IP routing is performed by all hosts, but most importantly by internetwork routers, which typically use either Interior Gateway Protocol s (IGPs) or External Gateway Protocol s (EGPs) to help make IP datagram forwarding decisions across IP connected networks. VERSION HISTORY IP is the common element found in today's public Internet . The current and most popular network layer protocol in use today is IPv4 ; this version of the protocol is assigned version 4. IPv4 was adopted by the United States Department Of Defense as MIL-STD-1778 . IPv6 is the proposed successor to IPv4 whose most prominent change is the addressing. IPv4 uses 32-bit addresses (~4 billion addresses) while IPv6 uses 128-bit addresses (~3.4×1038 addresses) Versions 0 through 3 were either reserved or unused; version 5 was used for an experimental stream protocol. Other version numbers have been assigned, usually for experimental protocols, but have not been widely used. SEE ALSO
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