/25 subnet mask (255.255.255.128)

A /25 network has 7 host bits, 128 addresses and 126 usable hosts. It is 1/2 of a /24.

/25 at a glance

Netmask: 255.255.255.128
Wildcard mask: 0.0.0.127
Hex mask: 0xFFFFFF80
Binary mask: 11111111.11111111.11111111.10000000
Host bits: 7
Total addresses: 128
Usable hosts: 126 (minus network and broadcast)
Compared with a /24: 1/2 of a /24

Calculate a /25 in the subnet calculator

Typical uses of a /25

A /25 splits a /24 in half (126 usable hosts each). It is a common way to separate two VLANs, such as servers and users, inside one /24 allocation.

Example /25 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /25
10.0.0.0/25	10.0.0.0	10.0.0.1	10.0.0.126	10.0.0.127	10.0.0.128/25	calculate
172.16.0.0/25	172.16.0.0	172.16.0.1	172.16.0.126	172.16.0.127	172.16.0.128/25	calculate
192.168.1.0/25	192.168.1.0	192.168.1.1	192.168.1.126	192.168.1.127	192.168.1.128/25	calculate
203.0.113.0/25	203.0.113.0	203.0.113.1	203.0.113.126	203.0.113.127	203.0.113.128/25	calculate

The /25 boundary falls in the fourth octet, so /25 networks start every 128 in that octet (2 per /24): 0, 128.

How many /25s fit

In a	Number of /25 subnets
/8	131,072
/16	512
/20	32
/24	2

What a /25 contains

Smaller block	How many
/26	2
/28	8
/29	16
/30	32
/32	128

Splitting a /25

Into	Subnets	Usable hosts each
/26	2	62
/27	4	30
/28	8	14
/29	16	6

/25 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/25 allowed?	Usable after reservation
AWS VPC	/16 to /28	Yes	123 (5 reserved)
Azure VNet	/2 to /29	Yes	123 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	124 (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.