Cisco CCNA (200-301)Network FundamentalsMedium
An engineer allocates the subnet 172.16.5.128/27 to a small office LAN. How many usable host addresses are available on this subnet?
- A30
- B126
- C14
- D62
Show answer & explanationAnswer & explanation
Correct answer: A. 30
A /27 mask (255.255.255.224) leaves 5 host bits (32-27=5). The total addresses are 2^5=32, minus 2 for the network and broadcast addresses, giving 30 usable host addresses.
Why the other options are wrong
- B. 126 usable hosts corresponds to a /25 subnet.
- C. 14 usable hosts corresponds to a /28 subnet, not /27.
- D. 62 usable hosts corresponds to a /26 subnet.
Usable Hosts Calculation
For any subnet, usable hosts = 2^(host bits) - 2, subtracting the network and broadcast addresses.
- /27 = 5 host bits = 30 usable hosts
- /28 = 4 host bits = 14 usable hosts
- /26 = 6 host bits = 62 usable hosts
Memory trick: Take 2 to the host bits, then evict 2 tenants (network & broadcast).