Internet

===Internet protocol===
[[Image:IP stack connections.svg|thumb|Conceptual data flow in a simple network topology of two hosts (''A'' and ''B'') connected by a link between their respective routers. The application on each host executes read and write operations as if the processes were directly connected to each other by some kind of data pipe. After the establishment of this pipe, most details of the communication are hidden from each process, as the underlying principles of communication are implemented in the lower protocol layers. In analogy, at the transport layer the communication appears as host-to-host, without knowledge of the application data structures and the connecting routers, while at the internetworking layer, individual network boundaries are traversed at each router.]]
The most prominent component of the Internet model is the Internet Protocol (IP). IP enables internetworking and, in essence, establishes the Internet itself. Two versions of the Internet Protocol exist, [[IPv4]] and [[IPv6]].

====IP Addresses====
[[File:An example of theoretical DNS recursion.svg|right|thumb|A DNS resolver consults three name servers to resolve the domain name user-visible "www.wikipedia.org" to determine the IPv4 Address 207.142.131.234.]]
For locating individual computers on the network, the Internet provides [[IP address]]es. IP addresses are used by the Internet infrastructure to direct internet packets to their destinations. They consist of fixed-length numbers, which are found within the packet. IP addresses are generally assigned to equipment either automatically via [[DHCP]], or are configured.

However, the network also supports other addressing systems. Users generally enter [[fully qualified domain name|domain name]]s (e.g. "en.wikipedia.org") instead of IP addresses because they are easier to remember; they are converted by the [[Domain Name System]] (DNS) into IP addresses which are more efficient for routing purposes.

====IPv4====
[[IPv4|Internet Protocol version 4]] (IPv4) defines an IP address as a [[32-bit]] number.<ref name="rfc791"/> IPv4 is the initial version used on the first generation of the Internet and is still in dominant use. It was designed to address up to ≈4.3&nbsp;billion (10<sup>9</sup>) hosts. However, the explosive growth of the Internet has led to [[IPv4 address exhaustion]], which entered its final stage in 2011,<ref>{{cite web|last=Huston |first=Geoff |title=IPv4 Address Report, daily generated |url=http://www.potaroo.net/tools/ipv4/index.html |access-date=20 May 2009 |url-status=live |archive-url=https://web.archive.org/web/20090401001902/http://www.potaroo.net/tools/ipv4/index.html |archive-date=1 April 2009 }}</ref> when the global IPv4 address allocation pool was exhausted.

====IPv6====
Because of the growth of the Internet and the [[IPv4 address exhaustion|depletion of available IPv4 addresses]], a new version of IP [[IPv6]], was developed in the mid-1990s, which provides vastly larger addressing capabilities and more efficient routing of Internet traffic. IPv6 uses 128 bits for the IP address and was standardized in 1998.<ref name=rfc1883>{{Cite IETF|rfc=1883|title=Internet Protocol, Version 6 (IPv6) Specification|author-link1=Steve Deering|author1=S. Deering|author2=R. Hinden|date=December 1995|publisher=Network Working Group}}</ref><ref name=rfc2460>{{Cite IETF|rfc=2460|title=Internet Protocol, Version 6 (IPv6) Specification|author-link1=Steve Deering|author1=S. Deering|author2=R. Hinden|publisher=Network Working Group|date=December 1998}}</ref><ref name=rfc8200>{{Cite IETF|rfc=8200|title=Internet Protocol, Version 6 (IPv6) Specification|author-link1=Steve Deering|author1=S. Deering|author2=R. Hinden|publisher=[[IETF]]|date=July 2017}}</ref> [[IPv6 deployment]] has been ongoing since the mid-2000s and is currently in growing deployment around the world, since Internet address registries ([[Regional Internet registry|RIRs]]) began to urge all resource managers to plan rapid adoption and conversion.<ref>{{cite web |url=https://www.arin.net/knowledge/about_resources/ceo_letter.pdf |title=Notice of Internet Protocol version 4 (IPv4) Address Depletion |access-date=7 August 2009 |archive-url=https://web.archive.org/web/20100107095025/https://www.arin.net/knowledge/about_resources/ceo_letter.pdf |archive-date=7 January 2010 }}</ref>

IPv6 is not directly interoperable by design with IPv4. In essence, it establishes a parallel version of the Internet not directly accessible with IPv4 software. Thus, translation facilities must exist for internetworking or nodes must have duplicate networking software for both networks. Essentially all modern computer operating systems support both versions of the Internet Protocol. Network infrastructure, however, has been lagging in this development. Aside from the complex array of physical connections that make up its infrastructure, the Internet is facilitated by bi- or multi-lateral commercial contracts, e.g., [[peering agreement]]s, and by technical specifications or protocols that describe the exchange of data over the network. Indeed, the Internet is defined by its interconnections and routing policies.

====Subnetwork====
[[File:Subnetting Concept-en.svg|thumb|300px|right|Creating a subnet by dividing the host identifier]]
A ''[[subnetwork]]'' or ''subnet'' is a logical subdivision of an [[IP network]].<ref name="rfc950">{{Cite IETF|rfc=950|publisher=[[IETF]]|author1=Jeffrey Mogul|author2=Jon Postel|author-link2=Jon Postel|title=Internet Standard Subnetting Procedure|date=August 1985}} Updated by RFC 6918.</ref>{{rp|1,16}} The practice of dividing a network into two or more networks is called ''subnetting''.

Computers that belong to a subnet are addressed with an identical [[most-significant bit]]-group in their IP addresses. This results in the logical division of an IP address into two fields, the ''network number'' or ''routing prefix'' and the ''rest field'' or ''host identifier''. The ''rest field'' is an identifier for a specific [[Host (network)|host]] or network interface.

The ''routing prefix'' may be expressed in [[Classless Inter-Domain Routing]] (CIDR) notation written as the first address of a network, followed by a slash character (''/''), and ending with the bit-length of the prefix. For example, {{IPaddr|198.51.100.0|24}} is the prefix of the [[Internet Protocol version 4]] network starting at the given address, having 24 bits allocated for the network prefix, and the remaining 8 bits reserved for host addressing. Addresses in the range {{IPaddr|198.51.100.0}} to {{IPaddr|198.51.100.255}} belong to this network. The IPv6 address specification {{IPaddr|2001:db8::|32}} is a large address block with 2<sup>96</sup> addresses, having a 32-bit routing prefix.

For IPv4, a network may also be characterized by its ''subnet mask'' or ''netmask'', which is the [[bitmask]] that when applied by a [[bitwise AND]] operation to any IP address in the network, yields the routing prefix. Subnet masks are also expressed in [[dot-decimal notation]] like an address. For example, {{IPaddr|255.255.255.0}} is the subnet mask for the prefix {{IPaddr|198.51.100.0|24}}.

Traffic is exchanged between subnetworks through routers when the routing prefixes of the source address and the destination address differ. A router serves as a logical or physical boundary between the subnets.

The benefits of subnetting an existing network vary with each deployment scenario. In the address allocation architecture of the Internet using CIDR and in large organizations, it is necessary to allocate address space efficiently. Subnetting may also enhance routing efficiency or have advantages in network management when subnetworks are administratively controlled by different entities in a larger organization. Subnets may be arranged logically in a hierarchical architecture, partitioning an organization's network address space into a tree-like routing structure.

====Routing====
Computers and routers use [[routing table]]s in their operating system to [[IP forwarding|direct IP packets]] to reach a node on a different subnetwork. Routing tables are maintained by manual configuration or automatically by [[routing protocol]]s. End-nodes typically use a [[default route]] that points toward an ISP providing transit, while ISP routers use the [[Border Gateway Protocol]] to establish the most efficient routing across the complex connections of the global Internet. The [[default gateway]] is the [[Node (networking)|node]] that serves as the forwarding host (router) to other networks when no other route specification matches the destination [[IP address]] of a packet.<ref>{{Cite web|url=https://www.lifewire.com/how-to-find-your-default-gateway-ip-address-2626072|title=How to Find Your Default Gateway IP Address|last=Fisher|first=Tim|website=[[Lifewire]]|access-date=25 February 2019|archive-date=25 February 2019|archive-url=https://web.archive.org/web/20190225162425/https://www.lifewire.com/how-to-find-your-default-gateway-ip-address-2626072|url-status=live}}</ref><ref>{{cite web|url=https://www.techopedia.com/definition/2184/default-gateway|title=Default Gateway|archive-url=https://web.archive.org/web/20201026160616/https://www.techopedia.com/definition/2184/default-gateway|archive-date=26 October 2020|website=techopedia.com|date=30 June 2020 }}</ref>