IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)
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The preceding topic illustrated how the fundamental division of the 32 bits in an IP address is into the network identifier (network ID) and host identifier (host ID). The network ID is used for routing purposes while the host ID uniquely identifies each network interface on the network. In order for devices to know how to use IP addresses on the network they must be able to tell which bits are used for each ID. However, the dividing line is not predefined. It depends on the type of addressing used in the network.
Understanding how these IDs are determined leads us into a larger discussion of the three main categories of IP addressing schemes. Each of these uses a slightly different system of indicating where in the IP address the host ID is found.
The original IP addressing scheme is set up so that the dividing line occurs only in one of a few locations: on octet boundaries. Three main classes of addresses, A, B and C are differentiated based on how many octets are used for the network ID and how many for the host ID. For example, class C addresses devote 24 bits to the network ID and 8 to the host ID. This type of addressing is now often referred to by the made-up word classful to differentiate it from newer classless scheme.
This most basic addressing type uses the simplest method to divide the network and host identifiers: the class, and therefore the dividing point, are encoded into the first few bits of each address. Routers can tell from these bits which octets belong to which identifier.
In the subnet addressing system, the two-tier network/host division of the IP address is made into a three-tier system by taking some number of bits from a class A, B or C host ID and using them for a subnet identifier. The network ID is unchanged. The subnet ID is used for routing within the different subnetworks that constitute a complete network, providing extra flexibility for administrators. For example, consider a class C address that normally uses the first 24 bits for the network ID and remaining 8 bits for the host ID. The host ID can be split into, say, 3 bits for a subnet ID and 5 for the host ID.
This system is based on the original classful scheme, so the dividing line between the network ID and full host ID is based on the first few bits of the address as before. The dividing line between the subnet ID and the sub-host ID is indicated by a 32-bit number called a subnet mask. In the example above, the subnet mask would be 27 ones followed by 5 zeroesthe zeroes indicate what part of the address is the host. In dotted decimal notation, this would be 255.255.255.224.
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