/3 subnet mask (224.0.0.0)

A /3 network has 29 host bits, 536,870,912 addresses and 536,870,910 usable hosts. It is 2,097,152 /24 networks.

/3 at a glance

Netmask: 224.0.0.0
Wildcard mask: 31.255.255.255
Hex mask: 0xE0000000
Binary mask: 11100000.00000000.00000000.00000000
Host bits: 29
Total addresses: 536,870,912
Usable hosts: 536,870,910 (minus network and broadcast)
Compared with a /24: 2,097,152 /24 networks

Typical uses of a /3

A /3 is an eighth of IPv4. The classful boundaries map onto it: 192.0.0.0/3 is all of the old Class C space and 224.0.0.0/3 is multicast plus Class E. Bogon and martian filter lists sometimes use these aggregates.

Well-known /3 blocks

192.0.0.0/3: The old classful Class C space (192-223)
224.0.0.0/3: Multicast plus reserved Class E space

Example /3 ranges

Network	Network address	First usable	Last usable	Broadcast	Next /3
0.0.0.0/3	0.0.0.0	0.0.0.1	31.255.255.254	31.255.255.255	32.0.0.0/3	calculate

The /3 boundary falls in the first octet, so /3 networks start every 32 in that octet (8 per address space): 0, 32, 64, 96, 128, 160, 192, 224.

What a /3 contains

Smaller block	How many
/16	8,192
/24	2,097,152
/26	8,388,608
/28	33,554,432
/29	67,108,864
/30	134,217,728
/32	536,870,912

Splitting a /3

Into	Subnets	Usable hosts each
/4	2	268,435,454
/5	4	134,217,726
/6	8	67,108,862
/7	16	33,554,430

/3 in AWS, Azure and Google Cloud

Platform	Allowed subnet sizes	/3 allowed?	Usable after reservation
AWS VPC	/16 to /28	No	-
Azure VNet	/2 to /29	Yes	536,870,907 (5 reserved)
Google Cloud VPC	/4 to /29	No	-

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.