DevNet Associate (DEVASC) v1.0Network FundamentalsMedium
A network engineer is configuring a new subnet for a department that requires 50 usable host IP addresses. The engineer needs to choose an appropriate subnet mask that minimizes wasted IP addresses while accommodating future growth. Which subnet mask should be used?
- A255.255.255.192
- B255.255.255.248
- C255.255.255.240
- D255.255.255.224
Show answer & explanationAnswer & explanation
Correct answer: A. 255.255.255.192
To accommodate 50 usable hosts, we need a block size that supports at least 52 addresses (50 hosts + network address + broadcast address). A /26 (255.255.255.192) provides 64 total addresses (2^6), with 62 usable hosts (64 - 2). This is the smallest block that meets the requirement.
Why the other options are wrong
- B. A /29 (255.255.255.248) provides 6 usable hosts, which is insufficient for 50 hosts.
- C. A /28 (255.255.255.240) provides 14 usable hosts, which is insufficient for 50 hosts.
- D. A /27 (255.255.255.224) provides 30 usable hosts, which is insufficient for 50 hosts.
Subnet Mask Calculation
Subnet mask calculation involves determining the number of host bits required to support a given number of hosts, which then defines the mask.
- Number of usable hosts = 2^h - 2 (where h is the number of host bits).
- The network and broadcast addresses are unusable for hosts.
- Smaller host bits mean a larger subnet mask (more 1s).
Memory trick: Variable Length Subnet Masks: 'Host bits Huddle, Network bits Nudge'