Definition

# 32-bit IP addressing

32-bit IP addressing is the IP address scheme used in Internet Protocol 4 (IPv6 uses a 128-bit system). In IPv4, an IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packets across the Internet.The 32-bit IP address (we have a separate definition of it with IP address) is often depicted as a dot address (also called dotted quad notation) - that is, four groups (or quads) of decimal numbers separated by periods. Here's an example:

``` 130.5.5.25
```

Each of the decimal numbers represents a string of eight binary digits. Thus, the above IP address really is this string of 0s and 1s:

``` 10000010.00000101.00000101.00011001
```

As you can see, we inserted periods between each eight-digit sequence just as we did for the decimal version of the IP address. Obviously, the decimal version of the IP address is easier to read and that's the form most commonly used.

Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address). IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here's how the IP address is divided:

``` <--Network address--><--Host address--> 130.5 . 5.25
```

If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address. Thus:

``` <--Network address--><--Subnet address--><--Host address--> 130.5 . 5 . 25
```

To simplify this explanation, we've divided the subnet into a neat eight bits but an organization could choose some other scheme using only part of the third quad or even part of the fourth quad.

Once a packet has arrived at an organization's gateway or connection point with its unique network number, it can be routed within the organization's internal gateways using the subnet number. The router knows which bits to look at (and which not to look at) by looking at a subnet mask, which is a screen of numbers that tells you which numbers to look at underneath.

This was last updated in March 2007

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