3.1 IPv4 Addressing Architecture and Classes

3.1 IPv4 Addressing Architecture and Classes

Quick Answer: An IPv4 address is a 32-bit binary identifier divided into four 8-bit octets expressed in dotted-decimal notation (such as 192.0.2.1). Historically, addresses were divided into five classes (Class A: 1.0.0.0–126.255.255.255, Class B: 128.0.0.0–191.255.255.255, Class C: 192.0.0.0–223.255.255.255, Class D: 224.0.0.0–239.255.255.255 for multicast, and Class E: 240.0.0.0–255.255.255.255 for experimental use). To conserve address space, RFC 1918 defines three private blocks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Special blocks include 127.0.0.0/8 (loopback), 169.254.0.0/16 (link-local/APIPA), and 255.255.255.255 (limited broadcast).

In TCP/IP internetworking, the Internet Protocol version 4 (IPv4) provides logical addressing and host identification at the Network Layer (Layer 3 of the OSI reference model). Whether configuring Nokia 7750 Service Router (SR) line interfaces or troubleshooting routed services across a service-provider backbone, a thorough mastery of IPv4 addressing fundamentals is mandatory.

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Structure of an IPv4 Address

An IPv4 address consists of exactly 32 binary bits. Because raw 32-bit binary strings (such as 11000000101010000000101000000001) are difficult for network engineers to read and manage, IPv4 addresses are formatted using dotted-decimal notation.

Octets and Positional Weight

The 32 bits are partitioned into four groups of 8 bits, with each 8-bit group termed an octet:

1. The octets are separated by decimal periods (dots).
2. Each octet has a decimal value range from 0 (00000000 in binary) to 255 (11111111 in binary).
3. The total mathematical address space encompasses $2^{32} = 4,294,967,296$ unique addresses.

To convert an octet from binary to decimal, calculate the sum of the positional weights for every bit position set to 1:

The Two-Level Hierarchy: Network ID and Host ID

Every IPv4 unicast address is divided into two distinct components:

• Network Identifier (Network ID): Specifies the physical or logical network to which the device belongs. Routers inspect the network portion of the destination IP address to determine the outgoing interface and next-hop router along the transit path.
• Host Identifier (Host ID): Identifies the specific host interface, router interface, server, or end device within that designated network segment.

In early TCP/IP networking, the boundary separating the network ID from the host ID was fixed by rigid class rules. In modern networking, that boundary is dynamically specified using subnet masks and prefix lengths.

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Historical Classful Addressing Architecture

Historical classful conventions, later expanded for multicast, categorized the address space into five classes: Class A, Class B, Class C, Class D, and Class E. The class of an address was determined strictly by the leading binary bits of the first octet.

Class A Networks

• Leading Bits: 0 (the most significant bit is always zero).
• First Octet Range: 1 to 126 in decimal (00000001 to 01111110 binary).
• Boundary Structure: The first octet serves as the Network ID (8 bits), while the remaining three octets represent the Host ID (24 bits).
• Default Subnet Mask: 255.0.0.0 (or /8 prefix length).
• Scale: Supports $2^7 - 2 = 126$ usable networks and $2^{24} - 2 = 16,777,214$ usable host addresses per network.
• Historical Intent: Assigned to very large organizations, government agencies, and major universities with millions of connected nodes.

Class B Networks

• Leading Bits: 10 (the first two bits are binary 1 and 0).
• First Octet Range: 128 to 191 in decimal (10000000 to 10111111 binary).
• Boundary Structure: The first two octets serve as the Network ID (16 bits), while the final two octets represent the Host ID (16 bits).
• Default Subnet Mask: 255.255.0.0 (or /16 prefix length).
• Scale: Supports $2^{14} = 16,384$ networks and $2^{16} - 2 = 65,534$ usable host addresses per network.
• Historical Intent: Assigned to medium-to-large enterprises, regional telecommunications providers, and large academic campuses.

Class C Networks

• Leading Bits: 110 (the first three bits are binary 1, 1, and 0).
• First Octet Range: 192 to 223 in decimal (11000000 to 11011111 binary).
• Boundary Structure: The first three octets serve as the Network ID (24 bits), while the final octet represents the Host ID (8 bits).
• Default Subnet Mask: 255.255.255.0 (or /24 prefix length).
• Scale: Supports $2^{21} = 2,097,152$ networks and $2^8 - 2 = 254$ usable host addresses per network.
• Historical Intent: Assigned to small businesses and local area networks (LANs).

Class D (Multicast)

• Leading Bits: 1110.
• First Octet Range: 224 to 239 in decimal (11100000 to 11101111 binary).
• Purpose: Reserved exclusively for multicast groups (one-to-many communication). Multicast addresses do not use subnet masks or host identifiers; instead, receiving hosts join specific multicast groups to listen for streaming media, routing updates (such as OSPF at 224.0.0.5), or control signaling.

Class E (Experimental / Reserved)

• Leading Bits: 1111.
• First Octet Range: 240 to 255 in decimal (11110000 to 11111111 binary).
• Purpose: Reserved by the IETF for experimental, research, and future development purposes. Addresses in this range are non-routable on the public Internet.

Classful Addressing Summary Table

Exam Tip: Notice that first octet values 0 and 127 are omitted from the usable Class A range. The 0.0.0.0/8 special-purpose block must not be confused with the default route 0.0.0.0/0, while network 127.0.0.0/8 is permanently dedicated to internal loopback testing.

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RFC 1918 Private IPv4 Address Space

Because the global pool of 4.29 billion IPv4 addresses was depleting rapidly in the early 1990s, the IETF published RFC 1918 (Address Allocation for Private Internets). RFC 1918 set aside three distinct blocks of IPv4 addresses that cannot be routed over the public Internet.

Any enterprise, service provider, or home user may freely deploy RFC 1918 addresses internally without obtaining permission or registration from the Internet Assigned Numbers Authority (IANA) or Regional Internet Registries (RIRs). RFC 1918 addresses are not globally unique and must not be advertised as public Internet destinations. Routers can route them internally; correct boundary filtering is an operational policy, not an automatic property of the address bits.

The Three RFC 1918 Address Blocks

Important Nuances Regarding the 172.16.0.0/12 Block

A frequent trap in networking examinations involves the exact boundary of the 172.16.0.0/12 private space:

• The private range extends from 172.16.0.0 through 172.31.255.255.
• Addresses with first and second octets below 172.16 (such as 172.15.1.1) or above 172.31 (such as 172.32.1.1 or 172.50.10.1) are outside that RFC 1918 block. This alone does not establish allocation, route advertisement, or reachability.

To connect devices operating with RFC 1918 private addresses to the public Internet, organizations deploy Network Address Translation (NAT) or Network Address Port Translation (NAPT) at the border router or firewall, translating private source IPs into globally routable public IPs.

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Reserved and Special-Purpose IPv4 Addresses

Beyond RFC 1918 private spaces, TCP/IP specifications reserve several specialized address ranges with distinct operational behaviors:

1. Loopback Address Block (127.0.0.0/8)

• RFC Standard: RFC 1122.
• Range: 127.0.0.0 to 127.255.255.255.
• Primary Address: 127.0.0.1 (localhost).
• Function: Packets addressed to 127.0.0.1 never leave the local device and are not transmitted onto any physical network link. The host operating system intercepts the packet at the network layer and loops it back up the local TCP/IP stack. Network administrators use ping 127.0.0.1 to test local IP processing. This does not test the physical NIC, cable, switch, or remote path.

2. Link-Local / APIPA Block (169.254.0.0/16)

• RFC Standard: RFC 3927.
• Range: 169.254.0.0 to 169.254.255.255.
• Function: Known as Automatic Private IP Addressing (APIPA) in operating systems like Microsoft Windows, link-local addressing allows a host to automatically assign itself an IP address when no static IP is configured and no Dynamic Host Configuration Protocol (DHCP) server responds on the local segment.
• Routing Boundary: Link-local traffic is strictly non-routable. Routers, including Nokia Service Routers, must never forward packets with source or destination addresses in the 169.254.0.0/16 range across router interfaces.

3. Limited Broadcast Address (255.255.255.255)

• Binary Representation: All 32 bits set to 1 (11111111.11111111.11111111.11111111).
• Function: Represents the limited broadcast (also called local wire broadcast). When a host transmits a packet to 255.255.255.255, the packet is delivered to every active host on the local physical network segment.
• Router Behavior: Routers block limited broadcast frames at Layer 3 boundaries. By default, a Nokia 7750 SR or 7705 SAR will never forward a packet addressed to 255.255.255.255 out another interface, thereby preventing broadcast storms from traversing the wider network.

4. Directed Broadcast Address

• Structure: The host portion of the address has all bits set to binary 1 within a specific subnet (for example, 192.168.1.255 in a /24 network).
• Function: Enables an off-subnet sender to transmit a packet that routers can route across intermediate hops until reaching the target destination subnet, where it is converted into a broadcast frame for all local hosts.
• Security: Modern routers disable forwarding of directed broadcasts by default to prevent Distributed Denial of Service (DDoS) reflection attacks (such as the historic Smurf attack).

5. Network Identifier (Network Address)

• Structure: The host portion of the address has all bits set to binary 0 (for example, 192.168.1.0 in a /24 network).
• Function: Identifies the entire network or subnetwork. It represents the route prefix in router routing tables (RIB) and forwarding tables (FIB). It is not a host address in a conventional LAN subnet. RFC 3021 /31 point-to-point links and /32 host routes have different semantics.

6. Unspecified Address (0.0.0.0/8)

• Function: The address 0.0.0.0 is used by a host that has not yet acquired an IP address during system bootstrap (for example, as the source IP address in a DHCP Discover packet). In router routing tables, 0.0.0.0/0 represents the default route (the gateway of last resort), matching all destination IP addresses not explicitly covered by more specific prefix entries.

Primary technical reference

Specification or Nokia manual, checked October 20, 2026. Examples are teaching scenarios, not captured router execution.