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ADVANCED COMPUTER NETWORKING

Lesson 6. Learning Outcome: Description of Classless Inter-Domain Routing (CIDR). ADVANCED COMPUTER NETWORKING. Why IPv4 IP address allocation is wasteful.

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ADVANCED COMPUTER NETWORKING

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  1. Lesson 6 Learning Outcome: Description of Classless Inter-Domain Routing (CIDR). ADVANCED COMPUTER NETWORKING © McLean 2006

  2. Why IPv4 IP address allocation is wasteful • Under the current 32-bit address scheme an organisation must select a network class that will provide enough IP addresses for their needs. • This means a Class A address could be assigned to organisations that require more than 65,536 address, though do not require up to 16, 777, 216! • Likewise Class B addresses are assigned to organisations that require more than 256 IP addresses but not anywhere near as many as 65,536. • Class C addresses are allocated to small networks with the potential for 256 addresses, even if they only require 20. • Therefore, a huge number of allocated IP addresses are not actually used. ADVANCED COMPUTER NETWORKING © McLean 2006

  3. Classless Inter-Domain Routing (CIDR) A solution to the problem of wasted IP address under the conventional system is CIDR. Essentially, it enables multiple Class C addresses to be combined, circumventing the “16,777,216 or 65,536 or 256 host addresses and nothing on between” Class A or B or C allocation method – hence why its called classless. This technique is also known as supernetting as it combines smaller component networks into one larger network. CIDR makes possible, which conventional allocation of blocks of IP addresses cannot, the allocation of parts of a class B or C block of IP addresses instead of the whole block. ADVANCED COMPUTER NETWORKING © McLean 2006

  4. Classless Inter-Domain Routing (CIDR) For example, if a business needs 1000 IP addresses it could get four 256-address Class Cs and combine them for a total of 1,024 host addresses. Under the conventional system it would have taken a whole Class B address and left most of it unused. It would now be capable of combining as many 256 IP address Class C networks as it required How is this actually achieved? ADVANCED COMPUTER NETWORKING © McLean 2006

  5. Classless Inter-Domain Routing (CIDR) Conventional system ADVANCED COMPUTER NETWORKING Class A – nnnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh Class B – nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh Class C – nnnnnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh © McLean 2006

  6. Classless Inter-Domain Routing (CIDR) ADVANCED COMPUTER NETWORKING © McLean 2006

  7. Classless Inter-Domain Routing (CIDR) CIDR uses network identifiers from 13 to 27 bits. Thus, blocks of addresses can be assigned to networks as small as 32 hosts or to those with over 500,000 hosts. Limitations Although CIDR is a solution, because the addresses are fragmented the job of routing packets becomes more complex and therefore less efficient. Also, routers have to be able to support the CIDR addressing system. IPv6 is generally regarded as the only real solution to the world running out of IP addresses ADVANCED COMPUTER NETWORKING © McLean 2006

  8. Classless Inter-Domain Routing (CIDR) How to calculate the Subnet Mask from a CIDR address Example CIDR address: 212.43.43.33/27 Write down 27 bits as 1 (one), and the rest (5) as 0 (zero): 11111111.11111111.11111111.11100000 Convert to decimal: 255.255.255.224 Remember that class A networks are followed by a /8, class B networks are followed by a /16, and class C networks are followed by a /24. ADVANCED COMPUTER NETWORKING © McLean 2006

  9. Classless Inter-Domain Routing (CIDR) • Activity • Calculate the Subnet Masks for the following CIDR addresses: • 192.30.250.0/18 • 130.125.2.0/13 • 193.214.123.0 /22 ADVANCED COMPUTER NETWORKING © McLean 2006

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