CompTIA Tech+ (FC0-U71)InfrastructureHard
A network administrator assigns a subnet mask of 255.255.255.240 to a small department LAN. How many usable host addresses are available on this subnet?
- A16
- B14
- C30
- D62
Show answer & explanationAnswer & explanation
Correct answer: B. 14
A subnet mask of 255.255.255.240 equals /28, leaving 4 host bits (32-28=4). Total addresses = 2^4 = 16, minus 2 (network and broadcast addresses) = 14 usable host addresses.
Why the other options are wrong
- A. 16 is the total address count before subtracting the network and broadcast addresses.
- C. 30 usable hosts corresponds to a /27 mask (255.255.255.224), not /28.
- D. 62 usable hosts corresponds to a /26 mask (255.255.255.192), not /28.
Subnet Mask & Host Calculation
A subnet mask defines how many bits are used for the network vs. host portion of an IP address, determining the number of usable addresses.
- 255.255.255.240 = /28, leaving 4 host bits
- Usable hosts = 2^(host bits) - 2
- Subtract 2 for the network address and broadcast address
Memory trick: Always subtract 2 — one for the network sign, one for the broadcast bullhorn.