A subnetwork, or subnet, is a logically visible, distinctly addressed part of a single Internet Protocol The Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP network.^[1][2] The process of subnetting is the division of a computer network A computer network, often simply referred to as a network, is a collection of computers and devices connected by communications channels that facilitates communications among users and allows users to share resources with other users. Networks may be classified according to a wide variety of characteristics. This article provides a general into groups of computers that have a common, designated IP address An Internet Protocol address is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A routing prefix.

Subnetting breaks a network into smaller realms that may use existing address space more efficiently, and, when physically separated, may prevent excessive rates of Ethernet Ethernet is a family of frame-based computer networking technologies for local area networks . The name came from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model as well as a common addressing format and Media Access Control at the Data Link Layer packet collision in a larger network. The subnets may be arranged logically in a hierarchical architecture, partitioning the organization's network address space (see also Autonomous System Within the Internet, an autonomous system is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet (cf. RFC 1930, Section 3)) into a tree-like routing structure. Routers A router is a device that interconnects two or more computer networks, and selectively interchanges packets of data between them. Each data packet contains address information that a router can use to determine if the source and destination are on the same network, or if the data packet must be transferred from one network to another. Where are used to interchange traffic between subnetworks and constitute logical or physical borders between the subnets. They manage traffic between subnets based on the high-order bit sequence (routing prefix) of the addresses.

A routing prefix is the sequence of leading (most-significant) bits A bit or binary digit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a digital device or other physical system that can usually exist in only two distinct states. These may be the two stable positions of an electrical switch, two distinct voltage or current levels allowed of an IP address An Internet Protocol address is a numerical label that is assigned to devices participating in a computer network, that uses the Internet Protocol for communication between its nodes. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A that precede both the portion of the address used as host identifier and, if applicable, the set of bits that designate the subnet number. Routing prefixes are expressed in CIDR notation CIDR notation is a compact specification of an Internet Protocol address and its associated routing prefix. Classless Inter-Domain Routing is an Internet Protocol (IP) address allocation and route aggregation methodology used within the Internet addressing architecture that replaced the IPv4 classful network organization of the IP address space, which uses the first address of a network followed by the bit-length of the prefix, separated by a slash (/) character. For example, 192.168.1.0/24 is the prefix of the IPv4 Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet. IPv4 is still by far the most widely deployed Internet Layer protocol. As of 2010[ network starting at the given address, having 24 bits allocated for the network number, and the rest (8 bits) reserved for host addressing. The IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 address specification 2001:db8::/32 is a large network for 2⁹⁶ hosts, having a 32-bit routing prefix.

In IPv4 networks, the routing prefix is traditionally expressed as a subnet mask, which is the prefix bit mask expressed in quad-dotted decimal representation In computer networking, dot-decimal notation is a method of writing binary numbers in octet grouped base-10 (decimal) numbers separated by dots (full stops). For example, 255.255.255.0 is the subnet mask for the 192.168.1.0/24 prefix.

All hosts within a subnet can be reached in one routing hop, implying that all hosts in a subnet are connected to the same link In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving digital information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver [two data terminal equipments ] and the interconnecting data telecommunication circuit. These are.

A typical subnet is a physical network served by one router, for instance an Ethernet Ethernet is a family of frame-based computer networking technologies for local area networks . The name came from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model as well as a common addressing format and Media Access Control at the Data Link Layer network, possibly consisting of one or several Ethernet segments or local area networks A local area network is a computer network covering a small physical area, like a home, office, or small groups of buildings, such as a school, or an airport. The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased, interconnected by network switches and network bridges or a Virtual Local Area Network (VLAN). However, subnetting allows the network to be logically divided regardless of the physical layout of a network, since it is possible to divide a physical network into several subnets by configuring different host computers to use different routers.

While improving network performance, subnetting increases routing complexity, since each locally connected subnet must be represented by a separate entry in the routing tables In computer networking a routing table, or Routing Information Base , is an electronic table (file) or database type object that is stored in a router or a networked computer. The routing table stores the routes (and in some cases, metrics associated with those routes) to particular network destinations. This information contains the topology of 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. Existing subnetting functionality in routers made the introduction of Classless Inter-Domain Routing Classless Inter-Domain Routing is a methodology of allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the prior addressing architecture of classful network design in the Internet with the goal to slow the growth of routing tables on routers across the Internet, and to help prevent the rapid seamless.

Network addressing

Computers and devices that are participating in a network such as the Internet each have a logical address. Usually this address is unique to each device and can either be dynamically The Dynamic Host Configuration Protocol is a computer networking protocol used by hosts (DHCP clients) to retrieve IP address assignments and other configuration information (from a dhcp server) or statically (by an administrator) configured. An address fulfills the functions of identifying the host and locating it on the network. It allows a device to communicate with other devices connected to the network. The most common network addressing scheme is Internet Protocol The Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP version 4 (IPv4 Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet. IPv4 is still by far the most widely deployed Internet Layer protocol. As of 2010[), but its successor, IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 is in early deployment Internet Protocol Version 6 is the next generation of the Internet Protocol that is currently in various stages of deployment on the Internet. It was designed as a replacement of the current version, IPv4, that has been in use since 1982 and is in the final stages of exhausting its unallocated address space stages. An IPv4 address consists of 32 bits, for human readability written in a form consisting of four octets Octet refers to an entity having exactly eight bits. As such, it is often used where the term byte might be ambiguous. For that reason, computer networking standards almost exclusively use octet. It is prominently used in Requests for Comments published by the Internet Engineering Task Force. The earliest example is RFC 635 from 1974. In France, separated by full stops A full stop or period (American English) (sometimes point or dot) is the punctuation mark commonly placed at the end of sentences (dots), called dot-decimal notation In computer networking, dot-decimal notation is a method of writing binary numbers in octet grouped base-10 (decimal) numbers separated by dots (full stops). An IPv6 address consists of 128 bits.

In order to facilitate routing Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network (Circuit switching) , electronic data networks (such as the Internet), and transportation networks. This article is concerned primarily with routing in electronic data networks a data packet across multiple networks, the address is divided into two parts:

• Network prefix: A contiguous group of high-order bits that are common among all hosts within a network.
• Host identifier: The remaining low-order bits of the address that are not designated in the network prefix. This part specifies a particular device in the local network.

The network prefix may be written in a form identical to that of the address itself. In IPv4, this is called the subnet mask of the address. For example, to specify the most-significant 18 bits of an address, i.e. in binary, 11111111.11111111.11000000.00000000, one writes this as 255.255.192.0.

An alternate form of specification of the routing prefix, is to simply count the number of bits in the routing prefix and append that number to the address with a slash (/) separator:

• 192.168.0.0, netmask 255.255.0.0
• 192.168.0.0/16

This latter notation is used preferentially in Classless Inter-Domain Routing Classless Inter-Domain Routing is a methodology of allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the prior addressing architecture of classful network design in the Internet with the goal to slow the growth of routing tables on routers across the Internet, and to help prevent the rapid and is called CIDR notation CIDR notation is a compact specification of an Internet Protocol address and its associated routing prefix. Classless Inter-Domain Routing is an Internet Protocol (IP) address allocation and route aggregation methodology used within the Internet addressing architecture that replaced the IPv4 classful network organization of the IP address space. In IPv6 this is the only acceptable form to denote routing prefixes.

The subnetting operation

The process of subnetting involves the separation of the network and subnet portion of an address from the host identifier. This is performed by a bitwise AND operation between the IP address and the subnet prefix or bit mask. The result yields the network address, and the remainder is the host identifier.

The following example is based on IPv4 networking. The operation may be visualized in a table using binary The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2. Owing to its straightforward implementation in digital electronic circuitry using logic gates, the binary system is used internally by all modern computers address formats.

In IPv4, subnet masks consist of 32 bits, usually a sequence of ones (1) followed by a block of 0s. The last block of zeros designate that part as being the host identifier.

Subnetting is the process of designating bits from the host portion and grouping them with the network portion. This divides a network into smaller subnets. The following diagram modifies the example by moving two bits from the host portion to the subnet number to form a smaller subnet:

Subnetting in IPv4 networks

Internet Protocol version 4 uses specially designated address formats to facilitate recognition of special address functionality. The first and the last subnets obtained by subnetting have traditionally had a special designation and, early on, special usage implications.^[3] In addition, IPv4 uses the all ones host address, i.e. the last address within a network, for broadcast transmission to all hosts on the link.

Subnet zero and the all-ones subnet

Previously discouraged

The first subnet obtained from subnetting has all bits in the subnet bit group set to zero . It is therefore called subnet zero^[4].

The last subnet obtained from subnetting has all bits in the subnet bit group set to one (1). It is therefore called the all-ones subnet.^[5]

Using these two subnets for addressing was discouraged at one point due to the confusion created by having a network and subnet with the same address. According to RFC 950, "It is useful to preserve and extend the interpretation of these special (network and broadcast) addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."^[6]

No longer discouraged

The practice of avoiding subnet zero and the all-ones subnet eventually became obsolete with RFC 1878, published in 1995. It explicitly states: "This practice is obsolete! Modern software will be able to utilize all definable networks."^[7]

Subnet and host counts

The number of subnetworks available, and the number of possible hosts in a network may be readily calculated. In the example (above) two bits were borrowed to create subnetworks, thus creating 4 (2²) possible subnets.

The RFC 950 specification reserves the subnet values consisting of all zeros (see above) and all ones (broadcast), reducing the number of available subnets by two. However, due to the inefficiencies introduced by this convention it was abandoned for use on the public Internet, and is only relevant when dealing with legacy equipment that does not implement CIDR. The only reason not to use the all-zeroes subnet is that it is ambiguous when the prefix length is not available. All CIDR-compliant routing protocols transmit both length and suffix. RFC 1878 provides a subnetting table with examples.

The remaining bits after the subnet are used for addressing hosts within the subnet. In the above example the subnet mask consists of 26 bits, leaving 6 bits for the host identifier. This allows for 64 combinations (2⁶), however the all zeros value and all ones value are reserved for the network ID and broadcast address respectively, leaving 62 addresses.

In general the number of available hosts on a subnet is 2ⁿ-2, where n is the number of bits used for the host portion of the address.

RFC 3021 specifies an exception to this rule when dealing with 31-bit subnet masks (i.e. 1-bit host identifiers). In such networks, usually point-to-point links Point-to-point is sometimes referred to as P2P or Pt2Pt.[citation needed] This usage of P2P is distinct from P2P referring to peer-to-peer file sharing networks, only two hosts (the end points) may be connected and a specification of network and broadcast addresses is not necessary.

Possible subnets within a /24 (traditional Class C) block are:

*only applicable on point-to-point links

Subnetting in IPv6 networks

The design of the IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 address space differs significantly from IPv4. The primary reason for subnetting in IPv4 is to improve efficiency in the utilization of the relatively small address space available, particularly to enterprises. No such limitations exist in IPv6, as the address space available even to end-users is large.

An IPv6 subnet always has 64 bits in its host portion. It therefore has a /64 routing prefix (the 64 most-significant bits). Although it is technically possible to use smaller subnets, they are impractical for local area networks because stateless address auto configuration of network interfaces (RFC 4862) requires a /64 address. IPv6 does not implement special address formats for broadcast traffic or network numbers, and thus all addresses in a subnet are valid host addresses.

The recommended allocation for an IPv6 customer site is an address space of 80 address bits (prefix /48), but it may be as small as 72 bits (/56 allocation) for a residential customer network.^[8] This provides 65,536 subnets for a site, or a minimum of 256 subnets for a residential network. Subnetting in IPv6 is used to route traffic between the global allocation spaces and within customer network between subnets and the larger Internet. Subnetting in IPv6 is also based on the concepts of Classless Inter-Domain Routing Classless Inter-Domain Routing is a methodology of allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the prior addressing architecture of classful network design in the Internet with the goal to slow the growth of routing tables on routers across the Internet, and to help prevent the rapid and the standard CIDR notation CIDR notation is a compact specification of an Internet Protocol address and its associated routing prefix. Classless Inter-Domain Routing is an Internet Protocol (IP) address allocation and route aggregation methodology used within the Internet addressing architecture that replaced the IPv4 classful network organization of the IP address space is used with IPv6 addresses.

See also

• IPv4 subnetting reference In the IPv4 address space certain address blocks are specially allocated or reserved for special uses such as loopback interfaces, private networks , and state-less autoconfiguration (Zeroconf, RFC 3927) of interfaces. Such addresses may be used without registration or allocation from Regional Internet Registries (RIRs). However, these address
• Classless Inter-Domain Routing Classless Inter-Domain Routing is a methodology of allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the prior addressing architecture of classful network design in the Internet with the goal to slow the growth of routing tables on routers across the Internet, and to help prevent the rapid
• Classful network Classful network is a term that is used to describe the network architecture of the Internet until around 1993. It divided the address space for Internet Protocol Version 4 into five address classes. Each class, coded by the first three bits of the address, defined a different size or type (unicast or multicast) of the network
• Autonomous System Within the Internet, an autonomous system is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet (cf. RFC 1930, Section 3)

References

1. ^ Jeffrey Mogul; Jon Postel Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards. He is known principally for being the Editor of the Request for Comment (RFC) document series, and for administering the Internet Assigned Numbers Authority until his death (August 1985). "RFC 950 - Internet Standard Subnetting Procedure". Internet Engineering Task Force (IETF). p. 1. http://tools.ietf.org/html/rfc950. "[...]subnets[...] of Internet networks[...] are logically visible sub-sections of a single Internet network."
2. ^ Jeffrey Mogul; Jon Postel Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards. He is known principally for being the Editor of the Request for Comment (RFC) document series, and for administering the Internet Assigned Numbers Authority until his death (August 1985). "RFC 950 - Internet Standard Subnetting Procedure". Internet Engineering Task Force (IETF). p. 16. http://tools.ietf.org/html/rfc950. "Subnet[:] One or more physical networks forming a subset of an Internet network. A subnet is explicitly identified in the Internet address."
3. ^ "Document ID 13711 - Subnet Zero and the All-Ones Subnet". Cisco Systems, Inc.. 2005-08-10. http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml. Retrieved 2010-04-25. "Traditionally, it was strongly recommended that subnet zero and the all-ones subnet not be used for addressing. [...] Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use."
4. ^ "Document ID 13711 - Subnet Zero and the All-Ones Subnet". Cisco Systems, Inc.. 2005-08-10. http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml. Retrieved 2010-04-23. "the first [...] subnet[...], known as subnet zero"
5. ^ "Document ID 13711 - Subnet Zero and the All-Ones Subnet". Cisco Systems, Inc.. 2005-08-10. http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml. Retrieved 2010-04-23. "[...] the last subnet[...], known as [...] the all-ones subnet"
6. ^ Jeffrey Mogul; Jon Postel Jonathan Bruce Postel was an American computer scientist who made many significant contributions to the development of the Internet, particularly with respect to standards. He is known principally for being the Editor of the Request for Comment (RFC) document series, and for administering the Internet Assigned Numbers Authority until his death (August 1985). "RFC 950 - Internet Standard Subnetting Procedure". Internet Engineering Task Force (IETF). p. 6. http://tools.ietf.org/html/rfc950. Retrieved 2010-04-23. "It is useful to preserve and extend the interpretation of these special addresses in subnetted networks. This means the values of all zeros and all ones in the subnet field should not be assigned to actual (physical) subnets."
7. ^ Troy Pummill; Bill Manning (December 1995). "RFC 1878 - Variable Length Subnet Table For IPv4". http://tools.ietf.org/html/rfc1878. "This practice is obsolete! Modern software will be able to utilize all definable networks." Note: RFC 1878 is not on the IETF standards track. It states of itself: "This memo does not specify an Internet standard of any kind.". It was published as "Category: Informational", and has since been demoted to "Category: Historic". RFC 950 is not marked as obsoleted as of today (2010-04-25). Which means that it is still current. It is therefore questionable whether a non-standards-track RFC may invalidate a non-obsoleted standards-track RFC.
8. ^ "IPv6 Addressing Plans". ARIN IPv6 Wiki. http://www.getipv6.info/index.php?title=IPv6_Addressing_Plans&oldid=2998. Retrieved 2010-04-25. "All customers get one /48 unless they can show that they need more than 65k subnets. [...] If you have lots of consumer customers you may want to assign /56s to private residence sites."

Further reading

• RFC 1812 Requirements for IPv4 Routers
• RFC 917 Utility of subnets of Internet networks
• RFC 1101 DNS Encodings of Network Names and Other Type
• Blank, Andrew G. TCP/IP Foundations Technology Fundamentals for IT Success. San Francisco, London: Sybex, Copyright 2004.
• Lammle, Todd. CCNA Cisco Certified Network Associate Study Guide 5th Edition. San Francisco, London: Sybex, Copyright 2005.
• Groth, David and Toby Skandier. Network + Study Guide, 4th Edition. San Francisco, London: Wiley Publishing, Inc., Copyright 2005.

External links

• "Understanding IP Addressing - Everything You Ever Wanted To Know" (PDF). 3COM. May 2001. pp. 76 pages. http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf. "1.08 MB"
• How to Subnet a Network
• IP Address Subnetting Tutorial
• Cisco-IP Addressing and Subnetting for New Users
• IP subnetting made easy
• Subnetworking at the Open Directory Project The Open Directory Project , also known as Dmoz (from directory.mozilla.org, its original domain name), is a multilingual open content directory of World Wide Web links. It is owned by Netscape, but it is constructed and maintained by a community of volunteer editors
• Free online program to Graphically Display IP Subnetworks

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