How IP Subnets Work
Internet protocol subnets are used to divide a network into smaller parts, so one part of the larger network is unable to access a different part directly. This provides much more efficient networking, and it also improves security by ensuring that computer users can only access the network areas for which they are authorized.
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IP Numerical Addressing
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Every computer and device on the Internet is assigned a numerical Internet address, in the format "nnn.nnn.nnn.nnn". "Nnn" is a number between 0 and 255 inclusive. For example, your computer at home may have the address 192.168.1.10, while your home Internet router has the address 192.168.1.1.
Reading IP Addresses
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You can think of an IP numerical address as a reverse form of a physical address. Your physical address starts with the local and ends with the national: 123 Main Street (local), Anytown (city), Massachusetts (state), USA (national). IP addresses do this in reverse. The first number designates a block of more than 16 million addresses, and each subsequent number narrows it down until it reaches the number for your router. In practice, every ISP has a block of addresses that it is allowed to assign. Large ISPs may have tens of millions, while smaller ISPs have far fewer.
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Subnet Masking
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A subnet is created by assigning a "subnet mask" in the router. It is called a "mask" because it acts as a barrier between computers inside the subnet and those outside; outside computers are invisible, or masked, from the subnet. Subnet masks are numbers, such as 255.255.255.0. The 255 is the binary number 11111111, which masks any numerical address that is different from the individual computer. A computer with the IP address 192.168.1.10, using a subnet mask of 255.255.255.0, could talk to computers using addresses starting with 192.168.1, and ending between 0 and 255, but it would not be able to contact computers at the addresses starting with 192.168.3 -- or any other variation of 192.168.1.
Reasons for Subnetting
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Internet devices, such as computers and routers, must move a great deal of data in small amounts of time; delays of more than a few milliseconds are considered to be too long. Subnets allow routers to move network traffic more quickly to the correct destination. A subnet also tells networked computers to only consider 255 (or fewer) addresses, even though a complete IP address allows for more than 2 billion locations; this substantially increases the networking speed.
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