What are public IP addresses, private IP addresses and reserved IP addresses?

In the age of the Internet, people who use the internet are likely familiar with IP addresses. However, IP addresses are further categorized into public addresses, private addresses, and reserved addresses, which many people may not fully understand. So, what exactly are public IP addresses, private IP addresses, and reserved IP addresses? What are the differences between them? Please see the following introduction.

Reserved IP addresses are a portion of the IP address range set aside by the International Internet Organization, IANA, as private IP address space or for special purposes like internal local area networks (LANs). For example, 255.255.255.255 is a reserved IP address. Networks using reserved addresses can only communicate internally and cannot interconnect with other networks.

A private network, also known as a local area network (LAN), is a small-scale network, typically forming in specific environments. This can be a network connecting multiple computers in a home, or a large-scale LAN within a school or company. Private networks can access the public internet via a router. Private IP addresses typically are 192.168.1.100, 172.18.8.101, 10.8.10.10, etc. In the IPv4 address protocol, three IP address ranges are reserved as private addresses for internal use by organizations.

From the definition of IANA, the private IP address is also classified as a reserved IP address.

The public network, also known as the internet, is a remote network that connects different regional LANs or metropolitan area networks (MANs). It typically spans a vast physical area, connecting multiple regions, cities, and countries to provide long-distance communication, forming a global internet. Every host connected to the public network has a public IP address, such as 221.234.147.249, 182.16.1.100. The vast majority of IP addresses are public and need to be registered with the International Internet Information Center.

IPv4 public addresses:

Apart from reserved IP addresses, all other IP addresses are public.

Public and private IP addresses can be converted using NAT (Network Address Translation) technology, allowing for address mapping. Generally, this is used to allow devices with private IP addresses to access external services via the gateway's public IP address. Different ports on the gateway's public IP address can also be mapped to various internal IP address devices.

Like IPv4, IPv6 also has public, private, and reserved IP addresses. IPv6's private addresses can be denoted using CIDR as fd00::/8, encompassing a much larger number of private IP addresses than IPv4.

To summarize, simply put, public IP addresses are the addresses of hosts on the entire internet. Private IP addresses are used within smaller networks, such as those at home, in companies, and on campuses, allocated by switches or routers. Reserved IP addresses have more specialized uses and typically cannot connect with other network IP addresses.