In 1993 the size of the routing tables started to

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In 1993, the size of the routing tables started to outgrow the capacity of routers Consequence: The Class-based assignment of IP addresses had to be abandoned
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CIDR Example CIDR notation of a network address: 192.0.2.0/18 "18" says that the first 18 bits are the network part of the address (and 14 bits are available for specific host addresses) The network part is called the prefix Assume that a site requires a network address with 1000 addresses - the network is assigned a continuous block of 1024 addresses with a 22-bit long prefix In CIDR Routing updates include the subnet mask Supports VLSM and Route Summarization
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CIDR: Prefix Size vs. Host Space CIDR Block Prefix # of Host Addresses /27 32 hosts /26 64 hosts /25 128 hosts /24 256 hosts /23 512 hosts /22 1,024 hosts /21 2,048 hosts /20 4,096 hosts /19 8,192 hosts /18 16,384 hosts /17 32,768 hosts /16 65,536 hosts
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CIDR and Address assignments Backbone ISPs obtain large block of IP addresses space and then reallocate portions of their address blocks to their customers. Example: Assume that an ISP owns the address block 206.0.64.0/18 , which represents 16,384 (2 32-18 =2 14 ) IP addresses Suppose a client requires 800 host addresses Assign a /22 block (512=2 9 <800<1024= 2 10 -> 32- 10 =22), e.g., 206.0.68.0/ 22 gives a block of 1,024 (2 10 ) IP addresses.
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