IPRangeExplorer

/21 vs /25 — Subnet Comparison

A /21 subnet is 16× larger than a /25. Every additional bit in the prefix halves the address space — the 4-bit difference between these two means /21 has 24 = 16 times as many addresses.

/21

2K IPs — building-scale subnet

Full reference →
Total IPs 2,048
Usable Hosts 2,046
Subnet Mask 255.255.248.0
Wildcard Mask 0.0.7.255

Typical Uses

  • →Large application tier
  • →Enterprise building network
  • →Kubernetes node pool subnet
/25

126 usable hosts — half a /24

Full reference →
Total IPs 128
Usable Hosts 126
Subnet Mask 255.255.255.128
Wildcard Mask 0.0.0.127

Typical Uses

  • →Public vs private half of a /24
  • →Department sub-segment
  • →Smaller cloud application subnets

Key Differences

16×
more IPs in /21 than /25
16
/25 subnets fit inside one /21
4
bits of difference in prefix length

How 16 /25 Subnets Divide a /21

Example using 10.0.0.0/21 as the parent block.

# CIDR Network First Usable Last Usable Broadcast Hosts
1 10.0.0.0/25 10.0.0.0 10.0.0.1 10.0.0.126 10.0.0.127 126
2 10.0.0.128/25 10.0.0.128 10.0.0.129 10.0.0.254 10.0.0.255 126
3 10.0.1.0/25 10.0.1.0 10.0.1.1 10.0.1.126 10.0.1.127 126
4 10.0.1.128/25 10.0.1.128 10.0.1.129 10.0.1.254 10.0.1.255 126
5 10.0.2.0/25 10.0.2.0 10.0.2.1 10.0.2.126 10.0.2.127 126
6 10.0.2.128/25 10.0.2.128 10.0.2.129 10.0.2.254 10.0.2.255 126
7 10.0.3.0/25 10.0.3.0 10.0.3.1 10.0.3.126 10.0.3.127 126
8 10.0.3.128/25 10.0.3.128 10.0.3.129 10.0.3.254 10.0.3.255 126
9 10.0.4.0/25 10.0.4.0 10.0.4.1 10.0.4.126 10.0.4.127 126
10 10.0.4.128/25 10.0.4.128 10.0.4.129 10.0.4.254 10.0.4.255 126
11 10.0.5.0/25 10.0.5.0 10.0.5.1 10.0.5.126 10.0.5.127 126
12 10.0.5.128/25 10.0.5.128 10.0.5.129 10.0.5.254 10.0.5.255 126
13 10.0.6.0/25 10.0.6.0 10.0.6.1 10.0.6.126 10.0.6.127 126
14 10.0.6.128/25 10.0.6.128 10.0.6.129 10.0.6.254 10.0.6.255 126
15 10.0.7.0/25 10.0.7.0 10.0.7.1 10.0.7.126 10.0.7.127 126
16 10.0.7.128/25 10.0.7.128 10.0.7.129 10.0.7.254 10.0.7.255 126

FAQ

What is the difference between /21 and /25?

A /21 has 2,046 usable hosts and a /25 has 126. The subnet masks differ: /21 uses 255.255.248.0 while /25 uses 255.255.255.128. Every additional bit in the prefix halves the number of addresses — so the 4-bit gap means /21 is exactly 16× larger.

How many /25 subnets fit in a /21?

Exactly 16 /25 subnets fit perfectly inside one /21 with no wasted space. To split a /21 into /25s, just increment the last 4 bits of the network address for each new subnet.

Which should I choose?

/21 is typically used for: Enterprise building / large app tier. /25 is better for: Half a /24 — public/private split. Choose the smallest prefix that comfortably fits your host count — over-allocating wastes address space, but under-allocating means painful renumbering later.

Related Comparisons

/24 vs /25 /25 vs /26 /21 vs /24