A server administrator needs to configure a server on a network that will have no more than 30 available IP addresses. Which of the following subnet addresses will be the MOST efficient for this network?

The goal is to find the most efficient subnet that provides at least 30 usable IP addresses. The number of usable host addresses in a subnet is calculated using the formula 2^h - 2, where 'h' is the number of host bits. A subnet mask of 255.255.255.224 is equivalent to a /27 prefix, leaving 5 bits for the host portion (32 total bits - 27 network bits). Applying the formula, 2^5 - 2 = 32 - 2 = 30 usable host addresses. This perfectly matches the requirement, making it the most efficient choice by minimizing wasted IP addresses.

A. 255.255.255.0: This /24 mask provides 254 usable addresses (2^8 - 2), which is highly inefficient for a 30-host network.
B. 255.255.255.128: This /25 mask provides 126 usable addresses (2^7 - 2), which is still excessive and wastes IP space.
D. 255.255.255.252: This /30 mask provides only 2 usable addresses (2^2 - 2), which is insufficient to meet the requirement of 30.

1. Internet Engineering Task Force (IETF). (1995). RFC 1878: Variable Length Subnet Table For IPv4. In the table provided in this document, the entry for the subnet mask 255.255.255.224 (CIDR /27) explicitly lists the "Usable IPs" as 30. This directly confirms the calculation.

2. Cisco. (2005). IP Addressing and Subnetting for New Users. Document ID: 13788. The section "Subnetting an IP Network" explains the formula for calculating available hosts. The example table under "Subnetting Scenario 2" demonstrates how a /27 mask yields 30 valid hosts per subnet.

3. Comer, D. E. (2009). Computer Networks and Internets (5th ed.). Pearson/Prentice Hall. Chapter 19, "IP: Addressing," Section 19.10, "Subnet Addressing," explains that the number of hosts is determined by the bits remaining after the network and subnet portions of the mask, and that two addresses (all 0s and all 1s in the host field) are reserved. This academic text provides the foundational principles for the calculation.