Subnet address

A subnetwork, also known as a subnet, is a division of a computer network. Subnetting a network allows you to break down a large network into smaller ones which results in reduced network overhead by dividing the parts which receive IP broadcasts.

In order to subnet, every machine must have be told its subnet mask that defines which parts of its IP address are allocated for the subnetwork ID and the host ID.

Network Masks

A network mask, also known as a subnet mask, netmask or address mask, is a 32-bit 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 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, a subnet could be refered to simply as 130.94.122.199/28.

Subnetworking Concept

IP addresses are broken down into three parts, the network part, the subnet part (often considered part of the network 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	0000	255.0.0.0
B	128.0.0.0	191.255.0.0	1000	255.255.0.0
C	192.0.0.0	223.255.255.0	1100	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 an 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:

• IPv4 subnetting reference
• CIDR
• classful network

External Links