/5 subnet mask (248.0.0.0)

A /5 network has 27 host bits, 134,217,728 addresses and 134,217,726 usable hosts. It is 524,288 /24 networks.

/5 at a glance

Netmask: 248.0.0.0
Wildcard mask: 7.255.255.255
Hex mask: 0xF8000000
Binary mask: 11111000.00000000.00000000.00000000
Host bits: 27
Total addresses: 134,217,728
Usable hosts: 134,217,726 (minus network and broadcast)
Compared with a /24: 524,288 /24 networks

Typical uses of a /5

A /5 holds 128 million addresses. It appears only in aggregated routes and in summary filters, for example when describing large address-registry blocks.

Example /5 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /5
0.0.0.0/5	0.0.0.0	0.0.0.1	7.255.255.254	7.255.255.255	8.0.0.0/5	calculate

The /5 boundary falls in the first octet, so /5 networks start every 8 in that octet (32 per address space): 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, ....

What a /5 contains

Smaller block	How many
/16	2,048
/24	524,288
/26	2,097,152
/28	8,388,608
/29	16,777,216
/30	33,554,432
/32	134,217,728

Splitting a /5

Into	Subnets	Usable hosts each
/6	2	67,108,862
/7	4	33,554,430
/8	8	16,777,214
/9	16	8,388,606

/5 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/5 allowed?	Usable after reservation
AWS VPC	/16 to /28	No	-
Azure VNet	/2 to /29	Yes	134,217,723 (5 reserved)
Google Cloud VPC	/4 to /29	Yes	134,217,724 (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.