/9 subnet mask (255.128.0.0)

A /9 network has 23 host bits, 8,388,608 addresses and 8,388,606 usable hosts. It is 32,768 /24 networks.

/9 at a glance

Netmask: 255.128.0.0
Wildcard mask: 0.127.255.255
Hex mask: 0xFF800000
Binary mask: 11111111.10000000.00000000.00000000
Host bits: 23
Total addresses: 8,388,608
Usable hosts: 8,388,606 (minus network and broadcast)
Compared with a /24: 32,768 /24 networks

Typical uses of a /9

A /9 is half of a /8. A common pattern is to split 10.0.0.0/8 into 10.0.0.0/9 and 10.128.0.0/9, for example one half for on-premises and one for cloud, so routes between them stay simple.

Example /9 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /9
10.0.0.0/9	10.0.0.0	10.0.0.1	10.127.255.254	10.127.255.255	10.128.0.0/9	calculate

The /9 boundary falls in the second octet, so /9 networks start every 128 in that octet (2 per /8): 0, 128.

How many /9s fit

In a	Number of /9 subnets
/8	2

What a /9 contains

Smaller block	How many
/16	128
/24	32,768
/26	131,072
/28	524,288
/29	1,048,576
/30	2,097,152
/32	8,388,608

Splitting a /9

Into	Subnets	Usable hosts each
/10	2	4,194,302
/11	4	2,097,150
/12	8	1,048,574
/13	16	524,286

/9 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/9 allowed?	Usable after reservation
AWS VPC	/16 to /28	No	-
Azure VNet	/2 to /29	Yes	8,388,603 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	8,388,604 (4 reserved)

Cloud providers reserve addresses in every subnet (first four and the last address (network, VPC router, DNS, future use, broadcast) on AWS), so the usable count there differs from the classic 2^n - 2.