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VPN-UPLINK - Encyclopedia: Subnetwork
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 Classless Inter-Domain Routing to be deployed fairly painlessly.

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, e.g. 255.128.0.0) or, less commonly, as an eight-digit hexadecimal number (e.g. FF.80.00.00 = 255.128.0.0).

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). For example, 192.0.2.96/28 indicates an IP address where the first 28 bits are used as the network address (same as 255.255.255.240).

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Subnetzwork".