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Basic Internet Architecture 4.2.1 Hierarchy and Aggregation The following issues are all closely related. IP address formats The association of IP addresses to endpoints How IP routing protocols establish appropriate paths? How routers make their next-hop forwarding decisions? For small networks, it might seem reasonable for every router to simply know the identity and location of every endpoint. In practice, this approach is unworkable, as real networks may have thousands or tens of thousands of endpoints. Considering the many millions of hosts on the Internet itself, it is clearly impossible to expect routers (having only finite memory and processing capacity) to know all possible destinations. The solution has been to introduce hierarchy into the IP address space one that maps closely related IP addresses onto topologically localised sets of actual IP endpoints. This hierarchy allows routers to carry summarised information for regions of the network further away from them, and increasingly more detailed information for closer regions of the network. Hierarchy also creates sparseness of address allocation (consequently, far less than 232 IP addresses can actually be allocated). 4.2.1.1 Class-Based Hierarchy The IPv4 unicast address space was originally blocked into three classes A, B and C (see Figure 4.8) [RFC791]. Specific combinations of an address most significant 3 bits identified an addresses class. The next most significant 7, 14 or 21 bits of the IP address represented a network number. The Internet itself (at the time known as ARPAnet) was modelled as a backbone (a network of routers) with multiple independent networks directly attached. Each attached network was assigned a specific class A, B or C network number. Endpoints (hanging off each network) had their IP addresses constructed from their network s class bit(s), network number bits and a locally significant value for the remaining 24, 16 or 8 host bits. A router could easily determine which part of a packet s destination address represented the destination network, because the class of an IP address was encoded in the top 3 bits. However, this class structure was particularly wasteful of address space. Many companies or institutions with more than 254 hosts had to obtain multiple class C networks (filling the backbones router tables) or a single class B (which would be barely utilised). In response, the Internet Engineering Task Force (IETF) developed Classless Inter Domain Routing (CIDR) in the early 1990s. Class Address Format in Binary Networks Hosts A 0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh 27 nets 224hosts B 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh 214 nets 216 hosts C 110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh 221 nets 28 hosts Figure 4.8 Early IPv4 space divided into fixed-size classes

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