Introducing Cisco Data Center Networking (DCICN) — Question 36

How many IP addresses are usable for a subnet that has the address 10.6.128.0/21?

Answer options

• A. 1024
• B. 2048
• C. 1022
• D. 2046
• E. 256

Correct answer: D

Explanation

The subnet mask /21 indicates that there are 11 bits available for host addresses, which means there are 2^11 = 2048 total addresses. However, two addresses are reserved (the network and broadcast addresses), resulting in 2048 - 2 = 2046 usable IP addresses. The other options do not account for the reserved addresses correctly or miscalculate the total number of addresses.