Public vs Private IP Addresses and NAT Explained: A Complete Guide

1. IP Address Classification

In network operations, IP addresses are the most frequently encountered element. Public (external) and private (internal) IPs are defined relative to a firewall: inside the firewall is the internal network, outside is the external network, which is essentially the public Internet.

Public addresses are reachable directly from the Internet, allowing both inbound and outbound communication, and are essential for hosting servers.

Class A : 10.0.0.0 – 127.255.255.255 – suited for large networks with many hosts but few LANs.

Class B : 128.0.0.0 – 191.255.255.255 – commonly used by large enterprises and government agencies.

Class C : 192.0.0.0 – 223.255.255.255 – typical for small companies, campuses, and research institutions.

Class D : 224.0.0.0 – 239.255.255.255 – reserved for special purposes such as multicast.

Class E : 240.0.0.0 – 255.255.255.255 – currently reserved.

2. Private Addresses

Private (internal) IP ranges are reserved for use within local networks and are not routable on the public Internet. The three private IPv4 blocks are:

10.0.0.0/8

172.16.0.0/12

192.168.0.0/16

Devices inside a LAN can use these addresses freely, but they must be translated to a public address to communicate outside. In many home networks, the 192.168.x.x range is chosen because a single Class C block can accommodate up to 254 devices, which is sufficient for typical household needs.

3. NAT (Network Address Translation)

Since private addresses cannot appear on the Internet, NAT translates internal IP addresses to one or a few public IP addresses, conserving the limited public address space. NAT works by maintaining a mapping table of source IP, source port, destination IP, and destination port.

Home routers typically use port‑forwarding (a form of NAT) to map multiple internal devices to a single public IP, each distinguished by a unique port number.

In summary, small‑scale networks usually adopt the 192.168.x.x private range, obtain a public address from the ISP (or a carrier‑grade NAT address), and rely on NAT to enable internet access for internal devices.