What is a /8 subnet? 16,777,216 IPs, split into smaller blocks

How this is calculated

In CIDR notation, /8 means the first 8 bits are the network part, leaving 24 bits for hosts (2^24 = 16,777,216). Broadcast traffic is the first limit. ARP and DHCP requests reach every host on a segment, so a flat network with millions of hosts would be swamped long before it ran out of addresses. Cloud subnets have a limit too. AWS only allows subnets from a /28 up to a /16, so a /8 has to be split before AWS can use any part of it. A /8 holds 256 /16 blocks, or 65,536 /24 blocks, and those are the sizes a planner works from.

Verdict

Treat a /8 as a planning block, not a network. Its mask, 255.0.0.0, leaves 24 host bits, so the block holds 16,777,216 addresses and 16,777,214 usable ones. One workable plan gives each region a /16 and each VLAN inside it a smaller block. Summary routes matter here. The core routers can advertise one route per region instead of one route for every VLAN. A /8 fits a very large organization that wants one private address plan across many sites. The block counts above show how much room that plan has before it runs out.