IPv4 Class A Addresses (1.0.0.0 – 126.255.255.255)
Learn about Class A IPv4 addresses with a /8 default mask. Understand the address range, binary structure, and why classful addressing has been replaced by CIDR.
Detailed Explanation
IPv4 Class A Addresses
Class A addresses use the first octet for the network ID and the remaining three octets for host IDs, providing a massive number of hosts per network.
Range and Structure
| Property | Value |
|---|---|
| Range | 1.0.0.0 – 126.255.255.255 |
| First octet | 1 – 126 |
| Default Mask | 255.0.0.0 (/8) |
| Networks | 126 |
| Hosts per network | 16,777,214 |
Note: 0.x.x.x is reserved for "this network" and 127.x.x.x is reserved for loopback.
Binary Identifier
Class A addresses always start with a 0 bit:
0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
│ │ │
Network Host (24 bits)
Examples
| Address | Network | Host |
|---|---|---|
| 10.0.0.1 | 10.0.0.0 | 0.0.0.1 |
| 44.128.5.10 | 44.0.0.0 | 0.128.5.10 |
| 100.64.0.1 | 100.0.0.0 | 0.64.0.1 |
Historical Context
In the early internet, organizations were assigned entire /8 blocks. Companies like Apple (17.0.0.0/8), Ford (19.0.0.0/8), and the US DoD (various /8 blocks) received these allocations. This led to massive address waste and contributed to IPv4 exhaustion.
CIDR Replaced Classful Addressing
Modern networking uses CIDR (Classless Inter-Domain Routing) instead of classes. A "Class A" network can be subnetted into thousands of smaller networks using variable-length subnet masks (VLSM).
Use Case
A network architect reviews legacy Class A allocations to reclaim unused address space, subnetting a /8 into hundreds of /24 networks for efficient utilization across data center racks.