CompTIA Network+ (N10-009)Networking ConceptsHard
An ISP allocates a customer the IPv6 prefix 2001:db8:1234:5670::/60. The network administrator plans to create separate /64 subnets for each VLAN within this allocation. How many /64 subnets are available?
- A32
- B16
- C8
- D64
Show answer & explanationAnswer & explanation
Correct answer: B. 16
The difference between the /60 prefix and the /64 subnet size is 4 bits (64 − 60 = 4). This yields 2^4 = 16 available /64 subnets within the /60 allocation.
Why the other options are wrong
- A. 32 would require a 5-bit difference, which does not match /60 to /64.
- C. 8 would result from only 3 borrowed bits (2^3), not the actual 4-bit difference.
- D. 64 would require a 6-bit difference, which is more than the actual /60-to-/64 gap.
IPv6 Subnetting Math
The number of subnets created when subdividing a larger IPv6 prefix into smaller fixed-size prefixes equals 2 raised to the power of the bit difference between the two prefix lengths.
- Formula: 2^(new prefix - original prefix)
- /64 is the standard subnet size for IPv6 LANs
- No need to subtract network/broadcast addresses like IPv4
Memory trick: Borrow 4 bits, get 16 slots — 2^4 always equals 16.