The 255.0.0.0 subnet mask.

The /8 subnet uses 255.0.0.0 as its subnet mask — meaning the first 8 bits of every address identify the network, and the remaining 24 bits identify the host within that network. That gives you 16.78M total addresses (16.78M usable on standard RFC math, after subtracting the network and broadcast addresses).

The wildcard mask — the bitwise inverse of the subnet mask — is 0.255.255.255. Wildcards are what Cisco access-control lists and OSPF area definitions use instead of subnet masks; the "1" bits mark "don't care" positions. For a /8, that leaves 24 don't-care host bits.

To find the network address for any IP in a /8 block, perform a bitwise AND between the IP and the subnet mask. To find the broadcast, OR the network address with the wildcard. Modern tools — like our subnet calculator — do this in microseconds, but the underlying mechanics are straightforward binary arithmetic.

A /8 block contains 16.7 million addresses. In the historical Class A scheme, /8 was an entire enterprise allocation. Today /8 ranges like 10.0.0.0/8 are reserved for private use (RFC 1918) and you'll see them as the parent block of an entire corporate or cloud-provider network — never as a single subnet.

Cloud-provider quirks matter at every prefix size: AWS and Azure reserve 5 IPs per subnet, GCP reserves 4, and OCI reserves 3. So a /8 on standard RFC math gives you 16.78M usable hosts, but on AWS or Azure that drops to 16.78M. The capacity-planning gap bites hardest at small prefixes (a /28 has 14 usable on paper, only 11 on AWS) but exists at every size. Our cloud-aware calculator applies the right math automatically.