A subnetwork, or subnet, is a logically visible subdivision of an IP network.[1] The practice of dividing a single network into two or more networks is called subnetting and the networks created are called subnetworks or subnets.
All 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" — either a computer, or a device, or specific network interface on a computer or device.
The routing prefix is expressed in CIDR notation. It is written as the first address of a network, followed by a slash character (/), followed by 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 network for 296 hosts, having a 32-bit routing prefix. In IPv4 the routing prefix is also specified in the form of the subnet mask, which is expressed in quad-dotted decimal representation like an address. For example, 255.255.255.0 is the network mask for the 192.168.1.0/24 prefix.
Traffic between subnetworks is routed with special devices called routers; they provide gateways between subnetworks. As logical or physical borders between the subnets, they can be configured to either allow communication between any two IP address on the separate subnets, or prevent it.
The benefits of subnetting 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.
Contents |
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 dynamically from a network server, statically 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, is in early deployment stages. An IPv4 address consists of 32 bits, for human readability written in a form consisting of four decimal octets separated by full stops (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 logically divided into two fields, a network field and the host 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, albeit no efficiency is gained. The rest "host" field 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 netmask, 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, used for both IPv4 and IPv6, counts the number of bits in the prefix and appends that number to the address with a slash (/) separator:
This notation was introduced with Classless Inter-Domain Routing (CIDR) and is called CIDR notation (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.[2] For IPv6, however, on-link determination is different in detail and requires the Neighbor Discovery Protocol (NDP).[3][4] 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.
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 (sub)network prefix. The result yields the network address or prefix, and the remainder is the host identifier.
An IPv4 network mask consists of 32 bits, a sequence of ones (1) followed by a block of 0s. The trailing block of zeros (0) designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address (192.168.5.130) and its associated /24 network mask (255.255.255.0). The operation is visualized in a table using binary address formats.
| Binary form | Dot-decimal notation | |
|---|---|---|
| IP address | 11000000.10101000.00000101.10000010 |
192.168.5.130 |
| Subnet mask | 11111111.11111111.11111111.00000000 |
255.255.255.0 |
| Network prefix | 11000000.10101000.00000101.00000000 |
192.168.5.0 |
| Host part | 00000000.00000000.00000000.10000010 |
0.0.0.130 |
The mathematical operation for calculating the network prefix is the binary and. The result of the operation yields the network prefix 192.168.5.0 and the host number 130.
Subnetting is the process of designating some high-order bits from the host part and grouping them with the network mask to form the subnet mask. This divides a network into smaller subnets. The following diagram modifies the example by moving 2 bits from the host part to the subnet mask to form a smaller subnet one fourth the previous size:
| Binary form | Dot-decimal notation | |
|---|---|---|
| IP address | 11000000.10101000.00000101.10000010 |
192.168.5.130 |
| Subnet mask | 11111111.11111111.11111111.11000000 |
255.255.255.192 |
| Network prefix | 11000000.10101000.00000101.10000000 |
192.168.5.128 |
| Host part | 00000000.00000000.00000000.00000010 |
0.0.0.2 |
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.[5] 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.
The first subnet obtained from subnetting has all bits in the subnet bit group set to zero (0). It is therefore called subnet zero.[6] 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.[7]
The IETF originally discouraged the production use of these two subnets at one point due to possible confusion of having a network and subnet with the same address.[8] The practice of avoiding subnet zero and the all-ones subnet was declared obsolete in 1995 by RFC 1878, an informational, but now "historic" (IETF jargon for historical) RFC.[9]
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.
| Network | Network (binary) | Broadcast address |
|---|---|---|
192.168.5.0/26 |
11000000.10101000.00000101.00000000 |
192.168.5.63 |
192.168.5.64/26 |
11000000.10101000.00000101.01000000 |
192.168.5.127 |
192.168.5.128/26 |
11000000.10101000.00000101.10000000 |
192.168.5.191 |
192.168.5.192/26 |
11000000.10101000.00000101.11000000 |
192.168.5.255 |
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, only two hosts (the end points) may be connected and a specification of network and broadcast addresses is not necessary.
A /24 network may be divided into the following subnets by increasing the subnet mask successively by one bit. This affects the total number of hosts that can be addressed in the /24 network (last column).
| CIDR notation | Network mask | Available subnets |
Usable hosts per subnet |
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 for point-to-point links
The design of the 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 RFC 4291 compliant subnet always uses IPv6 addresses with 64 bits for the host portion.[10] It therefore has a /64 routing prefix (128−64 = the 64 most significant bits). Although it is technically possible to use smaller subnets,[11] they are impractical for local area networks based on Ethernet technology, because 64 bits are required for stateless address auto configuration.[12] The Internet Engineering Task Force recommends the use of /64 subnets even for point-to-point links, which consist of only two hosts.[13]
IPv6 does not implement special address formats for broadcast traffic or network numbers,[14] and thus all addresses in a subnet are valid host addresses. The all-zeroes address is reserved as the Subnet-Router anycast address.[15]
The recommended allocation for an IPv6 customer site is an address space with an 80-bit (/48) prefix.[16] This provides 65536 subnets for a site. Despite this recommendation, other common allocations are /56 (72 bits) as well as /64 prefixes for a residential customer network.
Subnetting in IPv6 is based on the concepts of variable length subnet masking (VLSM) and the Classless Inter-Domain Routing methodology. It is used to route traffic between the global allocation spaces and within customer networks between subnets and the larger internet.