IPv4 Addresses

Classification of IPv4 Addresses

An IPv4 address is represented as 32 bits and is classified into several types depending on its purpose and allocation range. Historically, addresses were classified by class (A through E), but today classless addressing based on CIDR (Classless Inter-Domain Routing) is the mainstream approach. In addition, special-purpose address ranges are defined, such as global/private, loopback, link-local, and shared addresses.

Classful Addressing (Class A–E)

Classes A through C are defined as unicast addresses for hosts, and the boundary between the network portion and host portion (the classful mask) is determined by the pattern of the leading bits. Class D is reserved exclusively for multicast, and Class E is reserved for experimental use; neither is assigned to ordinary hosts.

Global Addresses (Public Addresses)

These are addresses whose uniqueness is guaranteed across the internet and that can be routed directly on the internet. They are allocated by IANA (Internet Assigned Numbers Authority) through the Regional Internet Registries (RIRs) to ISPs and organizations, and correspond to the ranges remaining after excluding special-purpose blocks such as private addresses and loopback addresses described below.

Private Addresses

These are address ranges reserved for use within local networks that are not directly connected to the internet, such as inside an organization. These addresses are not routed on the internet and can be reused independently by multiple organizations.

Loopback Address

This is an address reserved to refer to the host itself. Packets addressed to the loopback address are processed internally by the host without passing through a network interface. In IPv4, the entire 127.0.0.0/8 block is reserved for loopback use, although 127.0.0.1 is typically used as the representative address.

Link-Local Address

This is an address valid only within the same link (the same segment), used when a host automatically assigns itself an address (APIPA: Automatic Private IP Addressing) because it was unable to obtain one from a DHCP server or similar. Packets addressed to or originating from a link-local address are not forwarded beyond a router.

Shared Address Space

This is an address space reserved for use within an ISP’s own network in Carrier-Grade NAT (CGN / CGNAT) environments. It occupies a position similar to private addresses (RFC 1918), but was newly defined to avoid address collisions in cases where both the ISP’s equipment and customer-premises equipment use RFC 1918 addresses.

Related RFCs