CIDR Practice Question_
Duration: 3 min
This video lesson is available to enrolled students.
AI Summary
An AI-generated summary of this video lecture.
The lecture focuses on validating and representing a specific range of IP addresses as a Classless Inter-Domain Routing (CIDR) block. The instructor presents a problem involving the range 150.10.20.64 to 150.10.20.127 and systematically applies three specific rules to determine validity. He calculates the total number of addresses, verifies it is a power of two, and checks the divisibility of the starting address. Finally, he derives the subnet mask length by calculating the necessary host bits to complete the CIDR notation.
Chapters
0:00 – 2:00 00:00-02:00
The instructor introduces the problem statement on screen: 'Consider a block of IP Addresses ranging from 150.10.20.64 to 150.10.20.127'. He lists three rules for CIDR blocks: contiguity, power-of-two size, and divisibility of the first address. He calculates the block size using the formula 127 - 64 + 1, resulting in 64. He lists powers of 2 (2, 4, 8, 16, 32, 64) to confirm 64 is valid. He then checks the third rule by dividing the first address (64) by the block size (64), noting the remainder is zero. He concludes the block is valid. He emphasizes that the first address must be divisible by the size to ensure the host ID ranges from all 0s to all 1s.
2:00 – 3:00 02:00-03:00
The instructor proceeds to find the CIDR representation. He draws a 32-bit IP structure divided into Network ID and Host ID. Since the block size is 64, he determines that 6 bits are required for the host portion because 2^6 = 64. He subtracts these 6 host bits from the total 32 bits to find the network prefix length: 32 - 6 = 26. He writes the final CIDR notation as 150.10.20.64/26. He briefly sketches the binary values for the last octet to illustrate the range from 01000000 to 01111111. He explains that the network ID remains constant while the host ID varies across the block.
The lesson demonstrates a step-by-step algorithm for CIDR validation. First, calculate the total count of addresses. Second, verify the count is a power of 2. Third, ensure the starting address is a multiple of that count. Once validated, the host bits are derived from the log base 2 of the block size, and the prefix length is the remainder of 32 minus the host bits. This process ensures the IP block is aligned correctly within the address space.