什麼是Subnetwork

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The word subnetwork (subnet for short) has two related meanings. In the older and more general meaning, it meant one physical network of an internetwork. In the Internet Protocol (IP), a subnetwork (usually known as a subnet) is a division of a classful network. The rest of this article is about the second meaning.

Subnetting an IP network allows you to break down what appears (logically) to be a single 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 introduction of classful network numbers, it continued to be useful, as it reduced the number of entries in the Internet-wide routing table (by hiding information about all the individual subnets inside a site). As a side benefit, it also resulted in reduced network overhead, by dividing the parts which receive IP broadcasts.

　　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 decimal notation - four numbers from zero to 255 separated by periods, e.g. .

But in subnet masks only some of the numbers are allowed: 0,128,192,224,240,248,252,254,255

Less commonly, it can be represented as an eight-digit hexadecimal number (e.g. = ).

A shorter form, which is known as Classless Inter-Domain Routing (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, indicates an IP address where the first 28 bits are used as the network address (same as ).

　　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.

he Network ID is left out because it is designated for loopback and cannot be assigned to a network

　　Class E reserved

Subnetting is the process of allocating bits from the host portion as a network portion. For example, giving the class A network a subnet mask of would break it down into 256 sub-networks ( to ). Indicating that the first octet of the IP address shows the network address, the second one shows the subnet number and the last two show the host part. A bitwise AND operation of the host address with the subnet mask extracts the complete subnetwork address (see example below).

Subnet masks are not limited to whole octets, either. For example (or /15) is also a valid mask. Applied to a class A address this would create 128 subnetworks in intervals of two ( - , - , etc).