IP Routing In A Subnet Or Classless Addressing (CIDR) Environment
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There are three main categories of IP addressing: classful, subnetted classful, and classless. As we have already seen, the method used for determining whether direct or indirect delivery of a datagram is required is different for each type of addressing. The type of addressing used in the network also impacts how routers decide to forward traffic in an internet.
One of the main reasons why the traditional class-based addressing scheme was created was that it made both addressing and routing relatively simple. We must remember that IPv4 was developed in the late 1970s, when the cheap and powerful computer hardware we take for granted today was still in the realm of science fiction. For the internetwork to function properly, routers had to be able to look at an IP address and quickly decide what to do with it.
Classful addressing was intended to made this possible. There was only a two-level hierarchy for the entire internet: network ID and host ID. Routers could tell by looking at the first four bits which of the bits in any IP address were the network ID and which the host ID. Then they needed only consult their routing tables to find the network ID and see which router was the best route to that network.
The addition of subnetting to conventional addressing didn't really change this for the main routers on the internet, because subnetting is internal to the organization. The main routers handling large volumes of traffic on the Internet didn't look at subnets at all; the additional level of hierarchy that subnets represent existed only for the routers within each organization that chose to use subnetting. These routers, when deciding what to do with datagrams within the organization's network, had to extract not only the network ID of IP addresses, but also the subnet ID. This told them which internal physical network to send the datagram to.
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