/7 vs /9 — Subnet Comparison
A /7 subnet is 4× larger than a /9. Every additional bit in the prefix halves the address space — the 2-bit difference between these two means /7 has 22 = 4 times as many addresses.
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 4 /9 Subnets Divide a /7
Example using 10.0.0.0/7 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 |
| 3 | 11.0.0.0/9 | 11.0.0.0 | 11.0.0.1 | 11.127.255.254 | 11.127.255.255 | 8,388,606 |
| 4 | 11.128.0.0/9 | 11.128.0.0 | 11.128.0.1 | 11.255.255.254 | 11.255.255.255 | 8,388,606 |
FAQ
What is the difference between /7 and /9?
A /7 has 33,554,430 usable hosts
and a /9 has 8,388,606.
The subnet masks differ: /7 uses 254.0.0.0
while /9 uses 255.128.0.0.
Every additional bit in the prefix halves the number of addresses — so the 2-bit gap means
/7 is exactly 4× larger.
How many /9 subnets fit in a /7?
Exactly 4 /9 subnets fit perfectly inside one /7 with no wasted space. To split a /7 into /9s, just increment the last 2 bits of the network address for each new subnet.
Which should I choose?
Choose based on how many hosts you need. Use the hosts → prefix calculator on the homepage to find the right size for your requirements.