The most fundamental difference between public and private networks is their IP addressing scheme. Public networks, such as the global Internet, use globally unique and routable IP addresses assigned by regional internet registries. In contrast, private networks use specific address ranges defined in RFC 1918 (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16). These private addresses are not routable on the public Internet and are intended for use within a self-contained network. Communication between a private and public network requires a gateway device performing Network Address Translation (NAT).

A. Security is a design consideration for both network types; private networks are not inherently unsecured. In fact, their isolation provides a layer of security.

B. Cost is highly variable and depends on the network's scale and complexity, not on whether it is public or private.

D. This statement incorrectly reverses the definitions. Public networks are accessible to the general public, whereas private networks have restricted access.

1. Internet Engineering Task Force (IETF). (February 1996). RFC 1918: Address Allocation for Private Internets. Section 3, "Private Address Space." This document specifies the IP address blocks reserved for private networks. Retrieved from https://datatracker.ietf.org/doc/html/rfc1918

2. Cisco. (2023). IP Addressing: NAT Configuration Guide, Cisco IOS XE Gibraltar 16.12.x. Chapter: "NAT Overview," Section: "How NAT Works." This guide explains that NAT is used to translate "unregistered IP addresses" (private addresses) into "registered IP addresses" (public addresses) for Internet access.

3. Odom, W. (2019). CCNA 200-301 Official Cert Guide, Volume 1. Cisco Press. Chapter 13, "Implementing IP Addressing and Services," Section: "Private IP Networks." This chapter details the purpose and ranges of RFC 1918 private addresses for use inside an enterprise, separate from the public Internet.