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What are public IP addresses, private IP addresses and reserved IP addresses?
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.
What’s a reserved IP address?
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.
| Address block | Address range | Number of addresses | Scope | Description |
|---|---|---|---|---|
| 0.0.0.0/8 | 0.0.0.0–0.255.255.255 | 16,777,216 | Software | Current network (only valid as source address). |
| 10.0.0.0/8 | 10.0.0.0–10.255.255.255 | 16,777,216 | Private network | Used for local communications within a private network. |
| 100.64.0.0/10 | 100.64.0.0–100.127.255.255 | 4,194,304 | Private network | Shared address space for communications between a service provider and its subscribers when using a carrier-grade NAT. |
| 127.0.0.0/8 | 127.0.0.0–127.255.255.255 | 16,777,216 | Host | Used for loopback addresses to the local host. |
| 169.254.0.0/16 | 169.254.0.0–169.254.255.255 | 65,536 | Subnet | Used for link-local addresses between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from a DHCP server. |
| 172.16.0.0/12 | 172.16.0.0–172.31.255.255 | 1,048,576 | Private network | Used for local communications within a private network. |
| 192.0.0.0/24 | 192.0.0.0–192.0.0.255 | 256 | Private network | IETF Protocol Assignments. |
| 192.0.2.0/24 | 192.0.2.0–192.0.2.255 | 256 | Documentation | Assigned as TEST-NET-1, documentation and examples. |
| 192.88.99.0/24 | 192.88.99.0–192.88.99.255 | 256 | Internet | Reserved. IPv6 to IPv4 relay (included IPv6 address block 2002::/16). |
| 192.168.0.0/16 | 192.168.0.0–192.168.255.255 | 65,536 | Private network | Used for local communications within a private network. |
| 198.18.0.0/15 | 198.18.0.0–198.19.255.255 | 131,072 | Private network | Used for benchmark testing of inter-network communications between two separate subnets. |
| 198.51.100.0/24 | 198.51.100.0–198.51.100.255 | 256 | Documentation | Assigned as TEST-NET-2, documentation and examples. |
| 203.0.113.0/24 | 203.0.113.0–203.0.113.255 | 256 | Documentation | Assigned as TEST-NET-3, documentation and examples. |
| 224.0.0.0/4 | 224.0.0.0–239.255.255.255 | 268,435,456 | Internet | In use for IP multicast. (Former Class D network). |
| 240.0.0.0/4 | 240.0.0.0–255.255.255.254 | 268,435,455 | Internet | Reserved for future use. (Former Class E network). |
| 255.255.255.255/32 | 255.255.255.255 | 1 | Subnet | Reserved for the "limited broadcast" destination address. |
What’s a private IP address?
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.
| Class | Address range | Number of addresses | Number of networks |
|---|---|---|---|
| Class A | 10.0.0.0 ~ 10.255.255.255 | 16777216 | 1 Class A |
| Class B | 172.16.0.0 ~ 172.31.255.255 | 1048576 | 16 Class C |
| Class C | 192.168.0.0 ~ 192.168.255.255 | 65536 | 256 Class C |
What’s a public 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.
How do public and private IP addresses convert to each other?
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.
Does IPv6 also have public IP addresses, private IP addresses, and reserved IP addresses?
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.
What are the differences between public IP addresses, private IP addresses, and reserved IP addresses?
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.
