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

In the Internet age, I believe most people who can surf the Internet should not be very unfamiliar with IP addresses. However, IP addresses are classified as public addresses, private addresses, and reserved addresses, which may not be well understood by many people. So, what exactly are public IP addresses, private IP addresses and reserved IP addresses? What is the difference between them? Please see the following introduction.

Reserved IP addresses refer that the IANA keeps some IP address down in the range of IP address and make them used as private IP address space or for special purposes such as internal LAN. For example, 255.255.255.255 is a reserved IP address. A network using reserved addresses can only communicate internally and cannot interconnect with other networks.

Private network also called local area network（LAN）, is a small part of the Internet in terms of scope, and generally refers to the network formed under a specific environment. For example, a network formed by interconnecting multiple computers in a home, or a large LAN within schools and companies. The private network can access the public network through a router. The private IP addresses are usually 192.168.1.100, 172.18.8.101, 10.8.10.10 and so on. There are 3 IP address segments reserved in the IPv4 address protocol as private addresses for the internal use of the organization.

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

Public network, also called wide area network（WAN）, is a remote network that connects computers in different regional local area networks or metropolitan area networks for communication. It can usually span a large physical range, connect multiple regions, cities and countries to provide long-distance communication, forming global Internet. Each host connected to an extranet has a public IP address, such as 221.234.147.249, 182.16.1.100. The overwhelming majority of IP addresses are public addresses and need to apply for registration with the international Internet administrative agency.

IPv4 public addresses:

Except for reserved IP addresses, all other IP addresses are public IP addresses.

Public IP addresses and private IP addresses can be translated to each other by using NAT (Network Address Translation) technology to perform address mapping. Generally, it is mainly to let the private IP address device access the external services through the public IP address of the gateway. It is also possible to map multiple ports of the public IP address of the gateway to different private IP address devices.

There are two types of IP addresses: IPv4 and IPv6. Public IP addresses, private IP addresses and reserved IP addresses exist both of them. The private address of IPv6 can be expressed as fd00:: / 8 in the way of CIDR, and the quantity of private IP addresses in IPv6 is much larger than that of IPv4.

To sum up, simply put, public IP address refers to the address of the host on the whole Internet. Private IP address is the address of the LAN within a small area such as home, companies and campus, which is allocated by a switch or router. The use of reserved IP addresses is more special, and it cannot be connected with other network IP addresses in general.