A graphic representation of relationships and source of the various variables representing a chunk of C subnets

In computer networks, a subnetwork or subnet is a range of logical addresses within the address space that is assigned to an organization. Subnetting is a hierarchical partitioning of the network address space of an organization (and of the network nodes of an autonomous system) into several subnets. Routers constitute borders between subnets. Communication to and from a subnet is mediated by one specific port of one specific router, at least momentarily. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... A computer network is a system for communication among two or more computers. ... In computer networks, a logical address refers to a network layer address such as an IP address, an X.25/X.121 or IPX address. ... In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a network host, peripheral device, disk sector or other logical or physical entity. ... In the Internet, an autonomous system (AS) is a collection of IP networks and routers under the control of one entity (or sometimes more) that presents a common routing policy to the Internet. ... Cisco 1800 Router A router is a device that determines the proper path for data to travel between different networks, and forwards data packets to the next device along this path. ...

A typical subnet is a physical network served by one router, for instance an Ethernet network (consisting of one or several Ethernet segments or local area networks, 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. Local area network scheme A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings. ... A network switch is a computer networking device that connects network segments. ... A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. ... A virtual LAN, commonly known as a vLAN or as a VLAN, is a method of creating independent logical networks within a physical network. ...

The address to all nodes in a subnet starts with the same binary sequence, which is its network id and subnet id. In IPv4, the subnet may be identified by its base address and subnet mask. Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ...

Subnetting simplifies routing, since each subnet typically is represented by one row in the routing tables in each connected router. In computer networking a routing table is an electronic table (file) or database type object that is stored in a router or a networked computer. ...

Subnetting was originally introduced before the introduction of classful network addresses in IPv4, to allow a single larger network to have a number of smaller networks within it, controlled by several routers. Subnetting made Classless Inter-Domain Routing possible. Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4. ... Warning! This Article contains disinformation. ...

In order for computers to communicate on one network, they will all need their own IP address. However, if you have two or more networks, instead of purchasing different numbers for each network, subnetting provides the ability to easily divide one IP address range amongst all the hosts in your network. Subnetting is used in IP networks to break up larger networks into a smaller network. Subnetting involves manually calculating the bits of an IP address and taking them from the host side of bits and adding them to the network side.

Network address and logical address

The term network address sometimes refers to logical address, i.e. network layer address such as the IP address, and sometimes to the first address (the base address) of a classful address range to an organization. In computer networks, a logical address refers to a network layer address such as an IP address, an X.25/X.121 or IPX address. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... This article or section does not cite any references or sources. ... Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4. ...

Computers and devices that are part of an internetworking network such as the Internet all have a logical address. The network address is unique to that device and can either be dynamically or statically configured. This address allows the device to communicate with other devices connected to the network. The most common network addressing scheme is IPv4. The IPv4 network address consists of a 32 bit address divided into 4 octets and a subnet mask of like size. In order to facilitate the routing process the address is divided into two pieces, the network address and the host address. This works much like a postal address where the network address would represent the city and the host address would represent the street address. The subnet mask is used in conjunction with the network address to determine which part of the address is the network address and which part is the host address. (DHCP) is a set of rules used by a communications device such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment. ... In computer technology and networking, an octet is a group of 8 bits. ... It has been suggested that Routing protocol be merged into this article or section. ...

Binary subnet masks

While subnet masks are often represented in dot-decimal form, their use becomes clearer in binary. Looking at a network address and a subnet mask in binary, a device can determine which part of the address is the network address and which part is the host address. To do this, it performs a bitwise "AND" operation. Dot-decimal notation is a method of expressing octets using base-10 rather than hexadecimal. ... The binary numeral system, or base-2 number system, is a numeral system that represents numeric values using two symbols, usually 0 and 1. ... If two conditions are combined by and, they must both be true for the compound condition to be true as well. ...

Example

Subnet masks consist of a series of 1s in binary followed by 0s. The 1s designate that part of the address as being part of the network portion and the 0s designate that part as being part of the host address. Subnet masks do not have to fill a given octet. This allows a classful network to be broken down into subnets. A classful network is a network that has a subnet mask of 255.0.0.0, 255.255.0.0 or 255.255.255.0. Subnet masks can also be expressed in a shorter form, known as Classless Inter-Domain Routing (CIDR) notation, which gives the network number followed by a slash ("/") and the number of 'one' bits in the binary notation of the netmask (i.e. the number of relevant bits in the network number). For example, 192.0.2.96/24 indicates an IP address where the first 24 bits are used as network address (same as 255.255.255.0). Warning! This Article contains disinformation. ... Warning! This Article contains disinformation. ... The term notation can be used in several contexts. ... In mathematics, computer science, telecommunication etc. ... This article or section does not cite any references or sources. ...

IPv4 classes

Main article: IPv4 subnetting reference

IPv4 addresses are broken down into three parts: the network part, the subnet part (now often considered part of the network part, although originally it was part of the rest part), and the host part. Classful networks are obsolete, but the classful and classless networks will both be shown below: Certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback (RFC 1643), Private networks (RFC 1918), and Zeroconf (RFC 3927) usage, and are not available for allocation by Regional Internet Registries (RIRs). ...

The 127.0.0.1 network is left out because it is designated for loopback and cannot be assigned to a network.

Class D multicasting

Class E reserved

Subnetting is the process of allocating bits from the host portion as a network portion. The above example shows the bitwise "AND" process being performed on a classful network. The following example shows bits being borrowed to turn a classful network into a subnet. The allocation of production and consumption is a key element of any model of economics. ... If two conditions are combined by and, they must both be true for the compound condition to be true as well. ...

Subnetting

Example Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

In this example two bits were borrowed from the original host portion. This is beneficial because it allows a single network portion to be split into several smaller network portions. By design IPv4 addresses are limited in number and each classful network portion is capable of supporting a finite number of hosts. A /24 prefix, such as a classful C block, for example has space for 254 hosts. If a network were to be split into four parts using a classful prefix, four different /24 addresses would have to be used to serve those networks. Using the subnetting example above, if each subnetwork were to have 62 hosts or less (see below for maths), a single /24 prefix could be split up to service the entire network while wasting the fewest host addresses.

Subnets and host count

It is possible to determine the number of hosts and subnetworks available for any subnet mask. In the above example two bits were borrowed to create subnetworks. Each bit can take the value 1 or 0, giving 4 possible subnets (2² = 4)

According to the RFC 950 standard the subnet values consisting of all zeros and all ones are reserved, reducing the number of available subnets by 2. However due to the inefficiencies introduced by this convention it no longer used on the public Internet, and is only relevant when dealing with some legacy equipment that does not understand CIDR. The only reason not to use the all-zeroes subnet is that it is ambiguous when the exact prefix length is available. All CIDR-compliant routing protocols transmit both length and prefix. See RFC 1878 for a subnetting table with extensive examples. Subnet Zero is a computer networking term for the first subnet obtained after subnetting a network address. ...

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 address (32 − 26). This allows for 64 possible 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 can be calculated using the formula 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 host bit). According to the above rule a 31 bit mask would allow for 2¹ − 2 = 0 hosts. The RFC makes allowances in this case for certain types of networks (point-to-point) to disregard the network and broadcast address, allowing two host addresses to be allocated. Point-to-Point telecommunications is most recently (2003) referenced regarding wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. ...

Possible subnets for a /24 prefix (traditional Class C):

* only applicable on point-to-point links Point-to-Point telecommunications is most recently (2003) referenced regarding wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. ...

References

• RFC 950 Internet Standard Subnetting Procedure
• RFC 1812 Requirements for IPv4 Routers
• RFC 917 Utility of subnets of Internet networks
• RFC 1101 DNS Encodings of Network Names and Other Type
• RFC 1878 Variable Length Subnet Table For IPv4
• 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.

See also

• IPv4 subnetting reference
• Classless Inter-Domain Routing
• Classful network

Certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback (RFC 1643), Private networks (RFC 1918), and Zeroconf (RFC 3927) usage, and are not available for allocation by Regional Internet Registries (RIRs). ... Warning! This Article contains disinformation. ... Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4. ...

External links

• Understanding IP Addressing - Everything You Ever Wanted To Know (PDF) 76 pages. 3COM (May 2001). “1.08 MB”

• Subnetting Exerciser
• Subnet & CIDR Calculators
• Network Calculators
• IP Address Subnetting Tutorial
• Cisco-IP Addressing and Subnetting for New Users
• IP subnetting made easy
• Subnet Masking summary & definition