SUBNETWORK

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.
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.
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.
Subnetting simplifies routing, since each subnet typically is represented by one row in the routing tables in each connected router.
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.
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.

Contents

Network address and logical address

Binary subnet masks

IPv4 classes

Subnets and host count

References

See also

External links

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.
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.

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.
'Example'

	Dot-decimal Address	Binary
Full Network Address	192.168.5.10	11000000.10101000.00000101.00001010
Subnet Mask	255.255.255.0	11111111.11111111.11111111.00000000
Network Portion	192.168.5.0	11000000.10101000.00000101.00000000
Client Portion	0.0.0.10	00000000.00000000.00000000.00001010

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).

IPv4 classes

Main articles: 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:

Class	Leading bits	Start	End	Default Subnet Mask in dotted decimal	CIDR notation
A (CIDR /8)	0	0.0.0.0	127.255.255.255	255.0.0.0
B (CIDR /16)	10	128.0.0.0	191.168.255.255	255.255.0.0
C (CIDR /24)	110	192.0.0.0	223.255.255.255	255.255.255.0	/24
D (CIDR /28)	1110	224.0.0.0	239.255.255.255
E	1111	240.0.0.0	255.255.255.0

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.

'Example'

	Dot-decimal Address	Binary
Full Network Address	192.168.5.130	11000000.10101000.00000101.10000010
Subnet Mask	255.255.255.192	11111111.11111111.11111111.'11'000000
Network Portion	192.168.5.128	11000000.10101000.00000101.10000000

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 ('$2^\{2\}\; =\; 4$')

Network	Network (binary)	Broadcast address
192.168.5.0/26	11000000.10101000.00000101.'00'000000	192.168.5.63
192.168.5.64/26	11000000.10101000.00000101.'01'000000	192.168.5.127
192.168.5.128/26	11000000.10101000.00000101.'10'000000	192.168.5.191
192.168.5.192/26	11000000.10101000.00000101.'11'000000	192.168.5.255

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.
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 ('$2^\{6\}$'), 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 '$2^\{n\}-2$', 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 '$2^\{1\}-2\; =\; 0$' 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.
Possible subnets for a /24 prefix (traditional Class C):

CIDR notation	Network Mask	Available Networks	Available Hosts per network	Total usable hosts
/24	255.255.255.0	1	254	254
/25	255.255.255.128	2	126	252
/26	255.255.255.192	4	62	248
/27	255.255.255.224	8	30	240
/28	255.255.255.240	16	14	224
/29	255.255.255.248	32	6	192
/30	255.255.255.252	64	2	128
/31	255.255.255.254	128	2 ★	256

''
★ only applicable on point-to-point links

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

External links

★ Understanding IP Addressing - Everything You Ever Wanted To Know

★ 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

★ Online Subnet Calculators and IP Tools