Cisco CCNA (200-301)Network FundamentalsEasy
A network administrator must assign an address block to a LAN segment that needs to support 50 hosts, using the smallest subnet possible. Which subnet mask should be used?
- A255.255.255.128
- B255.255.255.192
- C255.255.255.224
- D255.255.255.240
Show answer & explanationAnswer & explanation
Correct answer: B. 255.255.255.192
A /26 mask (255.255.255.192) provides 2^6-2 = 62 usable host addresses, which is the smallest block that covers 50 hosts. A /27 only yields 30 usable hosts, which is too few.
Why the other options are wrong
- A. /25 gives 126 hosts, far more than needed, wasting addresses.
- C. /27 gives only 30 usable hosts, not enough for 50.
- D. /28 gives only 14 usable hosts, not enough for 50.
Host-based Subnetting
To find the smallest subnet for a required number of hosts, find the smallest power of 2 (minus 2 for network/broadcast) that is greater than or equal to the host requirement.
- Formula: 2^n - 2 >= required hosts
- /26 = 64 addresses, 62 usable
- Always subtract 2 for network and broadcast addresses
Memory trick: Two are lost every time: network and broadcast host slots.