Skip to content
W3S101

IPv4 Reserved Addresses

IPv4 - Reserved Addresses

Section titled “IPv4 - Reserved Addresses”

Certain IPv4 address ranges are reserved for special purposes and are not intended for general use on the public internet. These addresses cannot be routed globally and serve specific functions within networks.

Private IP Addresses (RFC 1918)

Section titled “Private IP Addresses (RFC 1918)”

These address blocks are designated for use in private networks, such as internal corporate LANs, home networks, or other isolated networks. They are not globally unique and can be reused by many different organizations. Routers on the public internet are configured to discard packets with these addresses in the source or destination fields.

The private IP address ranges are:

  • 10.0.0.0 to 10.255.255.255 (often referred to as the 10/8 block or 10.0.0.0/8)
  • 172.16.0.0 to 172.31.255.255 (the 172.16/12 block or 172.16.0.0/12)
  • 192.168.0.0 to 192.168.255.255 (the 192.168/16 block or 192.168.0.0/16)

To access the internet or other external networks, devices using these private IPs must typically go through a Network Address Translation (NAT) service, often provided by a router, which translates the private IP address to a public IP address. Proxy servers can also facilitate this.

The use of private IP addresses has been crucial in conserving the limited IPv4 public address space and delaying its exhaustion. Organizations can select a private range appropriate for their size and further subnet it internally to organize their network.

Loopback IP Addresses

Section titled “Loopback IP Addresses”

The address range 127.0.0.0 to 127.255.255.255 (127.0.0.0/8) is reserved for loopback purposes. The most common address used is 127.0.0.1, often referred to as ‘localhost’. When a host sends a packet to a loopback address, the operating system’s TCP/IP stack intercepts it and ‘loops it back’ to the same host without any data actually leaving the network interface card (NIC). This allows client and server processes on the same machine to communicate and is widely used for testing network applications and verifying that the TCP/IP software is installed and functioning correctly on the host.

Section titled “Link-local Addresses (APIPA)”

The range 169.254.0.0 to 169.254.255.255 (169.254.0.0/16), excluding the first and last /24 blocks (169.254.0.x and 169.254.255.x as per RFC 3927), is designated for link-local addressing. If a host is configured to obtain an IP address via DHCP but fails to contact a DHCP server, it may automatically assign itself an IP address from this range. This feature is known as Automatic Private IP Addressing (APIPA) in Windows systems. Link-local addresses allow for basic communication between devices on the same local network segment without manual configuration or a DHCP server. However, these addresses are not routable and cannot be used for communication beyond the local link (e.g., to access the internet).

Understanding these reserved address spaces is essential for network design, troubleshooting, and security. For a comprehensive list of special-purpose IPv4 addresses, consult the IANA IPv4 Special-Purpose Address Registry.