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A subnetwork, or subnet, is a logical, visible subdivision of an IP network.〔RFC 950, ''Internet Standard Subnetting Procedure'', J. Mogul, J. Postel (August 1985), page 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 a common, identical, most-significant bit-group in their IP address. This results in the logical division of an IP address into two fields, a network or routing prefix and the rest field or host identifier. The rest field is an identifier for a specific host or network interface. The routing prefix is expressed in CIDR notation. It is written as the first address of a network, followed by a slash character (''/''), and ending with the bit-length of the prefix. For example, 192.168.1.0/24 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. The IPv6 address specification 2001:db8::/32 is a large address block with 296 addresses, having a 32-bit routing prefix. For IPv4, a network is also characterized by its subnet mask, 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, 255.255.255.0 is the network mask for the 192.168.1.0/24 prefix. Traffic is exchanged (''routed'') between subnetworks with special gateways (routers) when the routing prefixes of the source address and the destination address differ. A router constitutes the 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 Classless Inter-Domain Routing (CIDR) and in large organizations, it is necessary to allocate address space efficiently. It 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. ==Network addressing and routing== Computers participating in a network such as the Internet each have at least one logical address. Usually this address is unique to each device and can either be configured automatically with the Dynamic Host Configuration Protocol (DHCP) by a network server, manually by an administrator, or automatically by stateless address autoconfiguration. An address fulfills the functions of identifying the host and locating it on the network. The most common network addressing architecture is Internet Protocol version 4 (IPv4), but its successor, IPv6, has been increasingly deployed since approximately 2006. An IPv4 address consists of 32 bits, for readability written in a form consisting of four decimal octets separated by dots, called dot-decimal notation. An IPv6 address consists of 128 bits written in a hexadecimal notation and grouping 16 bits separated by colons. For the purpose of network management, an IP address is divided into two logical parts, the network prefix and the host identifier or ''rest field''. All hosts on a subnetwork have the same network prefix. This routing prefix occupies the most-significant bits of the address. The number of bits allocated within a network to the internal routing prefix may vary between subnets, depending on the network architecture. While in IPv6 the prefix must consist of a set of contiguous 1-bits, in IPv4 this is not enforced, though there is no advantage to using non-contiguous 1-bits. The host part is a unique local identification and is either a host number on the local network or an interface identifier. This logical addressing structure permits the selective routing of IP packets across multiple networks via special gateway computers, called routers, to a destination host if the network prefixes of origination and destination hosts differ, or sent directly to a target host on the local network if they are the same. Routers constitute logical or physical borders between the subnets, and manage traffic between them. Each subnet is served by a designated default router, but may consist internally of multiple physical Ethernet segments interconnected by network switches or network bridges. The routing prefix of an address is written in a form identical to that of the address itself. This is called the network mask, or ''subnet mask'', of the address. For example, a specification of the most-significant 18 bits of an IPv4 address, 11111111.11111111.11000000.00000000, is written as 255.255.192.0. If this mask designates a subnet within a larger network, it is also called the ''subnet mask''. This form of denoting the network mask, however, is only used for IPv4 networks. The modern standard form of specification of the network prefix is CIDR notation, used for both IPv4 and IPv6. It counts the number of bits in the prefix and appends that number to the address after a ''slash'' (/) character separator: *192.168.0.0, netmask 255.255.255.0 is written as 192.168.0.0/24 *In IPv6, 2001:db8::/32 designates the address 2001:db8:: and its network prefix consisting of the most significant 32 bits. This notation was introduced with Classless Inter-Domain Routing (CIDR) in RFC 4632. In IPv6 this is the only acceptable form to denote network or routing prefixes. In classful networking in IPv4, prior to the introduction of CIDR, the network prefix could be directly obtained from the IP address, based on its highest order bit sequence. This determined the class (A, B, C) of the address and therefore the network mask. Since the introduction of CIDR, however, assignment of an IP address to a network interface requires two parameters, the address and its network mask. In IPv4, on-link determination for an IP address is given simply by the address and netmask configuration, as the address cannot be disassociated from the on-link prefix.〔RFC 1122, ''Requirements for Internet Hosts -- Communication Layers'', Section 3.3.1, R. Braden, IETF (October 1989)〕 For IPv6, however, on-link determination is different in detail and requires the Neighbor Discovery Protocol (NDP).〔RFC 4861, ''Neighbor Discovery for IP version 6 (IPv6)'', T. Narten et al. (September 2007)〕〔RFC 5942, '' IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes'', H. Singh, W. Beebee, E. Nordmark (July 2010)〕 IPv6 address assignment to an interface carries no requirement of a matching on-link prefix and vice versa, with the exception of link-local addresses. While subnetting may improve network performance in an organizational network, it increases routing complexity, since each locally connected subnet must be represented by a separate entry in the routing tables of each connected router. However, by careful design of the network, routes to collections of more distant subnets within the branches of a tree-hierarchy can be aggregated by single routes. Variable-length subnet masking (VLSM) functionality in commercial routers made the introduction of CIDR seamless across the Internet and in enterprise networks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「subnetwork」の詳細全文を読む スポンサード リンク
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