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The word subnetwork has two related meanings. In the older and more general meaning, it meant one physical network of an internet. In the Internet Protocol, a subnetwork, usually known as a subnet, is a division of a computer network. The rest of this article is about the second meaning.
Subnetting a network allows you to break down a large network into smaller ones. It was originally introduced before the introduction of classful network numbers in IPv4, to allow a single site to have a number of local area networks. Even after the network classes, it continued to be useful as it reduced the number of entries in the Internet-wide routing table. As a side benefit, it also results in reduced network overhead, by dividing the parts which receive IP broadcasts.
In order to subnet, every machine must be told its subnet mask, which defines what part of its IP address is allocated for the subnetwork ID, and what part for the host ID on that subnetwork. The fact that all hosts already used masks allowed CIDR to be deployed fairly painlessly.
Network masks
A network mask, also known as a subnet mask, netmask or address mask, is a bitmask used to tell how much of an IP address identifies the subnetwork the host is on and how much identifies the host.
Subnet masks are usually represented in the same representation used for addresses themselves; in IPv4, dotted quad notation (four numbers from zero to 255 separated by periods) or, less commonly, as an eight-digit hexadecimal number.
A shorter form, which is known as CIDR notation, 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). Using this notation, in IPv4 a subnet could be referred to simply as 130.94.122.199/28.
Subnetworking concept 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. There are three classes of IP address which determine how much is which. | Class | Start | End | First bits | Mask in dotted decimal | | A | 1.0.0.0 | 126.0.0.0 | 0 | 255.0.0.0 | | B | 128.0.0.0 | 191.255.0.0 | 10 | 255.255.0.0 | | C | 192.0.0.0 | 223.255.255.0 | 110 | 255.255.255.0 |
Subnetting is the process of allocating bits from the host portion as a network portion. For example, giving a class A address a subnet mask of 255.255.0.0 would break it down into 255 sub-networks. Indicating that the second octet of the IP address show the subnet number and the last two show the host part.
Subnet masks are not limited to whole octets, either. For example 255.128.0.0 (or /9) is also a valid mask. Applied to a class A address this would create 128 subnetworks in intervals of two (1.2.0.1 - 1.3.0.1, 1.4.0.1 - 1.5.0.1, etc).
Nonstandard subnets
Subnet zero Technically illegal, but still useable is the extreme first subnetwork. For example, subnet 1.0.0.0 with a subnet address of 255.255.0.0. The problem with this subnet is that the unicast address for the subnet is the same as the unicast address for the entire class A network.
Interleaved subnets It has been proposed that there might be some advantage (eg. against malicious individuals) in making the network IP addresses more unpredictable through the use of a less standard subnet mask. This is done by using subnet masks such as 201.35.98.105 which do not cleanly separate the address into the two sections but instead interleave them. This is to be used by MAGI Technologies Ltd. in their server suite, in conjunction with a DHCP server which assigns addresses in a relatively unpredictable manner. It is not yet known whether any router, ethernet switch or network hub will process these addresses. Such technology would also have to be used behind a NAT layer because they do not conform with any address pool assigned by IANA.
See also
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