IP Subnetting Examples
IP stands for Internet Protocol, which is the formal set of rules that governs communication between computers on the Internet. IP subnetting refers to the process of dividing a single physical network into two or more smaller logical networks, each of which shares an IP network address.
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IP Addresses and Classes
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An IP address consists of 32 binary digits, or bits, arranged into four groups of 8 bits, known as octets. Each octet represents a decimal number between 0 and 255, so IP addresses are usually written as four decimal numbers separated by periods, in what is known as dotted quad notation. An IP address is divided into two main portions, known as the network portion and the host portion. IP addresses can be divided into three classes, A, B, and C, and the class of an IP address determines where the boundary between the between the network portion and the host portion lies.
Example
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To illustrate the process of IP subnetting, imagine you have a wide area network consisting of three interconnected networks, each in a different geographical location, with 50 hosts each. You’ve been allocated the Class C network address, 192.168.123.0, which means that you can use IP addresses between 192.168.123.1 and 192.168.1.254. Note that the 192.168.123.0 and 192.168.123.255 addresses can’t be used for individual hosts because host numbers of all zeros or all ones are invalid (255 is equivalent of 1111111 in binary notation).
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Borrowing Bits
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If all 150 hosts were on a single network, you could allocate IP addresses to 254 hosts. However, your hosts are on three separate physical networks, so to reduce the number of IP addresses you need you divide, or subnet, your network. To do this, you effectively “borrow” some of the bits usually used for the host portion of an IP address and use them for the network portion of the address instead.
Subnet Mask
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To subnet the network, you apply another 32-bit number, known as a subnet mask, to the IP address to divide it into network and host portions. In this case, if you apply the subnet mask 255.255.255.192 -- or 1111111.11111111.1111111.11000000 in binary notation -- you create four subnets, each capable of supporting 62 hosts each. The first two bits of the last octet are used for the network portion of the address, so you create the additional networks, 0 (00000000 in binary), 64 (01000000), 128 (10000000) and 192 (11000000). The remaining 6 bits of the last octet can be used for host addresses on the individual subnets.
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References
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