/11 vs /12 — Subnet Comparison

Q: What is the difference between /11 and /12?

A /11 subnet has 2,097,150 usable host addresses, while a /12 has 1,048,574. A /11 is 2× larger than a /12 — exactly 2 /12 subnets fit inside a single /11.

Q: How many /12 subnets fit in a /11?

Exactly 2 /12 subnets fit inside a /11. Each /12 has 1,048,574 usable hosts, and together they cover all 2,097,152 IP addresses in the /11.

A /11 subnet is 2× larger than a /12. Every additional bit in the prefix halves the address space — the 1-bit difference between these two means /11 has 2¹ = 2 times as many addresses.

/11

Full reference →

Total IPs 2,097,152

Usable Hosts 2,097,150

Subnet Mask 255.224.0.0

Wildcard Mask 0.31.255.255

/12

1 million IPs — the 172.16/12 range

Full reference →

Total IPs 1,048,576

Usable Hosts 1,048,574

Subnet Mask 255.240.0.0

Wildcard Mask 0.15.255.255

Typical Uses

→RFC 1918 Class B private range (172.16.0.0/12)
→Large enterprise segments

Key Differences

2×

more IPs in /11 than /12

/12 subnets fit inside one /11

bit of difference in prefix length

How 2 /12 Subnets Divide a /11

Example using 10.0.0.0/11 as the parent block.

#	CIDR	Network	First Usable	Last Usable	Broadcast	Hosts
1	10.0.0.0/12	10.0.0.0	10.0.0.1	10.15.255.254	10.15.255.255	1,048,574
2	10.16.0.0/12	10.16.0.0	10.16.0.1	10.31.255.254	10.31.255.255	1,048,574

FAQ

What is the difference between /11 and /12?

A /11 has 2,097,150 usable hosts and a /12 has 1,048,574. The subnet masks differ: /11 uses 255.224.0.0 while /12 uses 255.240.0.0. Every additional bit in the prefix halves the number of addresses — so the 1-bit gap means /11 is exactly 2× larger.

How many /12 subnets fit in a /11?

Exactly 2 /12 subnets fit perfectly inside one /11 with no wasted space. To split a /11 into /12s, just increment the last 1 bit 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.