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Global Internet
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  1. Global Internet Computer Networks: Global Internet

  2. NSFNET backbone Stanford ISU BARRNET MidNet ■ ■ ■ regional Westnet regional regional Berkeley PARC UNL KU UNM NCAR UA Global Internet Figure 4.24 The tree structure of the Internet in 1990 Computer Networks: Global Internet

  3. Global Internet • Each provider network is regional and a single autonomous system (AS) • Major issues are: • Scalability of routing • Address utilization (running out of IP addresses • Hierarchy is used to improve scalability. Computer Networks: Global Internet

  4. Subnetting • Assigning one network number per physical network uses up IP address too fast! • More network numbers also increases forwarding table size. • The idea is to take a single IP network number and allocate the IP addresses with that network number to several physical networks which are referred to as subnets. • The subnets need to be close to each other for routing purposes. Computer Networks: Global Internet

  5. Subnetting • The mechanism by which a single network nubmer can be shared among multiple networks involves configuring all the nodes on each subnet with a subnet mask. • The subnet mask enables introduction of a single subnet number which provides for another level of hierarchy into the IP address. • All hosts on a given subnet are configured with the same mask, i.e., there is one subnet mask per subnet. Computer Networks: Global Internet

  6. Subnetting Figure 4.25 Subnet Addressing Computer Networks: Global Internet

  7. Subnetting Figure 4.26 An Example of Subnetting Computer Networks: Global Internet

  8. Classless Routing (CIDR) • Classless interdomain routing (CIDR) addresses: • The growth of backbone routing tables • The potential for 32-bit IP address space to be exhausted • The problem is with the Class B numbers (64K addresses) • Give out appropriate number of Class C addresses in 256 address chunks. • But this increases routing table entries for a single AS! Computer Networks: Global Internet

  9. Classless Routing (CIDR) • CIDR helps us to aggregate routes by breaking up rigid boundaries between classes. • Hand out Class C addresses in contiguous blocks by address. • Make it so the addresses share a common prefix => allocate Class C networks as a power of 2. • We need a protocol that understands these rules, e.g., BGP! • Network numbers are represented by (length,value) where length is the length of the prefix {similar to a mask}. Computer Networks: Global Internet

  10. Classless Routing (CIDR) Figure 4.27 Route Aggregation with CIDR Computer Networks: Global Internet