Steps to Subnetting a Class
Subnetting a class refers to breaking up, or segmenting, a single Class A, Class B or Class C computer network into multiple logical networks. Theoretically, a Class A network is capable of supporting 2^24, or 16,777,216, Internet Protocol addresses per network. A Class B network can support 2^16 or 65,536 IP addresses and a Class C network is capable of supporting 2^8 or 256 IP addresses. However, IP addresses are in short supply, so subnetting provides a method of stretching the finite number available.
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Subnets
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The first step in subnetting a class is to decide how many subnets you need. If your network is a Class C network consisting of three routers -- networking devices that forward units of data known as packets between networks -- each connected by serial links, you will need two subnets: one for each serial link. You also need one subnet for the network interface on each router, so you would need five subnets in total.
IP Address
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An IP address consists of a host portion, which identifies an individual computer, or host, on an IP network and a network portion, which identifies a specific IP network. To subnet a class, you need an addressing scheme that identifies each subnet with the original, larger network. You cannot extend the size of an IP address beyond 32 bits, so you need to “borrow” bits from the host portion to create an identifier for each subnet. On a Class C network, you have the bits 00000000 to play with.
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Borrowing Bits
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The next step in subnetting a class is to determine the number of bits you must borrow from the host portion of the IP address to create the required number of subnets. You can do this by using the formula # of subnets = 2^n – 2, where n represents the number of bits borrowed. If you borrow a total of three bits, the total number of usable subnets is 2^3 - 2, or 6, which is actually one more than you need for the network described above.
Subnet Mask
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A subnet mask is used to indicate which portion of an IP address refers to the subnet and which portion refers to the host. The default subnet mask for a Class C network is 255.255.255.000, which means that only the last eight bits, or octet, are available to identify subnets. To create six subnets, three bits must be borrowed from the host portion or, in other words, switched to 1 for subnetting, while the remaining bits are switched to 0 for host addressing. The binary value of the last octet is therefore 11100000 or, in decimal notation, 128+64+32 = 244, which gives a new subnet mask of 255.255.255.224 after borrowing three bits. To determine subnet addresses, subtract the subnet mask from 256 to determine the first subnet and keeping adding this number to itself until you reach the subnet mask value.
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References
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