15.3.1 IPv4 Addressing

IPv4 uses a 32-bit address, providing approximately 4.3 billion unique addresses. An IPv4 address is normally written as four decimal numbers separated by periods, with each number representing one byte (eight bits) of the address. This notation, known as dotted-decimal notation, is considerably easier for people to read than a 32-bit binary value. For example, 192.168.10.25 represents the binary address 11000000.10101000.00001010.00011001

Although routers process addresses in binary form, dotted-decimal notation is almost universally used for configuration and documentation. Conceptually, every IPv4 address consists of two parts:

a network identifier, which identifies the destination network; and
a host identifier, which identifies an individual device within that network.

This hierarchical structure allows routers to make forwarding decisions efficiently. Rather than maintaining a route to every individual computer connected to the Internet, routers simply forward packets toward the destination network. Once the packet reaches that network, the final router delivers it to the appropriate host.

The division between the network and host portions of an address is determined by the network prefix, commonly represented using Classless Inter-Domain Routing (CIDR) notation. For example, 192.168.10.0/24 indicates that the first 24 bits identify the network, while the remaining 8 bits identify individual hosts within that network. A /24 network therefore supports up to 256 address values (254 usable host addresses).

CIDR replaced the earlier class-based addressing system and allows address blocks of almost any practical size to be allocated. This greatly improves address utilization while reducing the size of Internet routing tables.

Large organizations commonly divide their allocated address space into smaller subnets. Subnetting improves network management, enhances security, and reduces unnecessary broadcast traffic by partitioning a large network into several smaller logical networks. Conversely, multiple smaller networks may be combined into a single summarized route through route aggregation, allowing Internet routers to exchange far fewer routing entries.

As the Internet expanded rapidly during the 1990s, it became clear that the 32-bit IPv4 address space would eventually become exhausted. Several techniques—including CIDR, route aggregation, and Network Address Translation (NAT)—significantly extended the useful life of IPv4, but they could not remove the fundamental limitation imposed by the finite address space. This ultimately led to the development of IPv6, described in the following section.