Cisco Certified Support Technician (CCST) NetworkingNetworking FundamentalsMedium
A network administrator is designing an IPv4 addressing scheme for a small office network. The network needs to support 25 hosts, and the administrator wants to minimize the waste of IP addresses. Which of the following subnet masks would be MOST appropriate for this requirement?
- A255.255.255.240
- B255.255.255.0
- C255.255.255.192
- D255.255.255.224
Show answer & explanationAnswer & explanation
Correct answer: D. 255.255.255.224
To support 25 hosts, we need to find the smallest power of 2 that is greater than or equal to (25 + 2) (2 for network and broadcast addresses). 2^5 = 32. This means 5 host bits are required. A 27-bit subnet mask (32 - 5 = 27) provides 30 usable IP addresses (2^5 - 2 = 30), which is sufficient for 25 hosts with minimal waste. The subnet mask for /27 is 255.255.255.224.
Why the other options are wrong
- A. 255.255.255.240 (/28) provides only 14 usable hosts (2^4 - 2 = 14), which is insufficient for 25 hosts.
- B. 255.255.255.0 (/24) provides 254 usable hosts, which is excessive for 25 hosts and wastes many IP addresses.
- C. 255.255.255.192 (/26) provides 62 usable hosts, still more than needed and wastes addresses.
IPv4 Subnet Mask
A 32-bit number that divides an IP address into network and host portions, allowing for efficient allocation and management of IP addresses within a network.
- Determines the size of a subnet.
- Consists of consecutive '1's for the network portion and '0's for the host portion.
- Expressed in dotted-decimal format or CIDR notation (e.g., /24).
Memory trick: Hosts Need Bits, Bits Make Masks