Internet

====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.