The /8 prefix: 16,777,216 addresses in one block
A /8 pins only the first octet and lets the remaining 24 bits roam: 16,777,216 addresses behind the mask 255.0.0.0. It is the largest block size anyone discusses in practice — there are only 256 possible /8s in the entire IPv4 space, and every one of them was allocated decades ago.
In the classful era this was a "Class A" network, handed to the likes of IBM, Apple, and the US Department of Defense. That heritage explains why 10.0.0.0/8 became the biggest of the three RFC 1918 private ranges: a single /8 gives one organization more internal addresses than it could ever plausibly number.
| Prefix length | /8 |
| Subnet mask | 255.0.0.0 |
| Wildcard mask | 0.255.255.255 |
| Total addresses | 16,777,216 |
| Usable hosts | 16,777,214 |
Who actually runs a /8
Almost nobody operates a public /8 as one flat network anymore; the original allocations were carved up and reassigned long ago. Where whole /8s still matter day to day is private addressing: enterprises standardize on 10.0.0.0/8 and subnet it internally, and carriers received 100.64.0.0/10 for CGNAT because even a private /8 is not always enough at their scale.
Cloud providers slice their virtual networks out of 10.0.0.0/8 as well. When AWS or Azure assigns your VPC a 10.x range, you are living inside a sliver of that one /8, walled off from every other customer's sliver by software-defined networking rather than by any route table you would recognize.
Subnetting inside a /8
A /8 contains 256 /16s, 65,536 /24s, or over four million /30s. The sane pattern is hierarchical: divide the /8 by region into /12s or /16s, divide those by site, then hand out /24s and smaller to individual VLANs. Done right, your route aggregation mirrors the split and the core tables stay small enough to actually debug.
CIDR Calculator
Run the math on 10.0.0.0/8 — or any other block
Frequently asked questions
How many IP addresses are in a /8?
16,777,216 in total (2^24 host bits), of which 16,777,214 are usable after subtracting the network and broadcast addresses.
What is the subnet mask of a /8?
255.0.0.0. Only the first octet is pinned; the other three octets are entirely host bits.
Is a /8 the same as a Class A network?
Yes — Class A was the classful name for blocks where the first octet alone identified the network. CIDR retired the class system in 1993, but the /8 kept the size and the reputation.
Can you still get a public /8 today?
No. All 256 /8s were allocated by IANA before the IPv4 exhaustion era, mostly in the 1980s and early 1990s. What remains of public IPv4 has been handed out in /24-sized crumbs for years.