/8 vs /9 — Subnet Comparison
A /8 subnet is 2× larger than a /9. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /8 has 21 = 2 times as many addresses.
16 million IPs — Class A network
Typical Uses
- →Entire Class A private range (10.0.0.0/8)
- →Large ISP or carrier allocations
- →Enterprise-wide addressing plan
8.3 million IPs — half a /8
Typical Uses
- →Large regional ISP allocation
- →Half of a Class A block
- →Aggregated routing prefix
Key Differences
How 2 /9 Subnets Divide a /8
Example using 10.0.0.0/8 as the parent block.
| # | CIDR | Network | First Usable | Last Usable | Broadcast | Hosts |
|---|---|---|---|---|---|---|
| 1 | 10.0.0.0/9 | 10.0.0.0 | 10.0.0.1 | 10.127.255.254 | 10.127.255.255 | 8,388,606 |
| 2 | 10.128.0.0/9 | 10.128.0.0 | 10.128.0.1 | 10.255.255.254 | 10.255.255.255 | 8,388,606 |
FAQ
What is the difference between /8 and /9?
A /8 has 16,777,214 usable hosts
and a /9 has 8,388,606.
The subnet masks differ: /8 uses 255.0.0.0
while /9 uses 255.128.0.0.
Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means
/8 is exactly 2× larger.
How many /9 subnets fit in a /8?
Exactly 2 /9 subnets fit perfectly inside one /8 with no wasted space. To split a /8 into /9s, just increment the last 1 bit of the network address for each new subnet.
Which should I choose?
/8 is typically used for: Entire private Class A or ISP allocation. /9 is better for: Large regional allocations. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.