When the subnet mask is applied to an IP address the result is the definition of some number of bits for use in identifying the subnetwork on which the device resides. For any given masking, the subnet bit value of all zeros and the value of all ones is not used to specify a device. RFC 950 "Internet Standard Subnetting Procedure" discusses these issues in detail.
To calculate the number of available subnets you use the formula: 2^n -2, where n is the number of bits used in the mask. In the case where three bits were used as the subnet portion of an address you would have: 2^3 - 2 = 6 combinations available. In reality, there are 8 combinations for a 3-bit pattern. Because of the restriction on using all zeros or all ones, the resulting number of legal subnets that can be uniquely identified with three bits is 6.
It should be noted that BSD 4.3 UNIX lets the administrator select zeros or ones to indicate a broadcast destination. In current practice zeros are never used. RFC 1118 and RFC 1009 both discuss these issues in more detail.
Here is an outline of the requirements for special-purpose subnet numbers. The reference to NETWORK means the network portion of the IP address (As defined by the Class A, B, or C address class range). SUBNET refers to that portion (those bits) in the address that are defined as the subnet portion (by the sub net/address mask). HOST means the remaining bits in the address (not included in the network or subnetwork portion).
|If You See This:||It Means This:|
|Entire IP address is all zeros||As a source address: "This host"
As a destination address: "I don't know the correct IP address to use so I'll use all zeros" This was also the early form of the IP destination address which meant "BROADCAST" but this usage is now considered obsolete.
|Entire IP address is all ones.||This is called a LIMITED BROADCAST. This is a broadcast to all hosts on the current subnet. A router will NOT forward this type of broadcast to other networks.|
|NETWORK = A valid network number.
SUBNET = A valid subnet number.
HOST = All ones.
|This is called a DIRECTED BROADCAST. This is a broadcast to all hosts on the specified network and subnetwork. A router WILL forward this frame for broadcast on the specified destination network and subnetwork.|
|127.x.x.x||This is a local loopback address. Frames sent to this address will be looped back and returned to the sending application without actually being sent onto the network.|
|0.x.x.x, 128.0.x.x, 191.255.x.x, 129.0.0.x, 223.255.255.x||These address constructions are reserved by the Network Information Center.|
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