Last reviewed: May 2026
A /20 subnet uses 20 bits for the network portion of an IPv4 address, leaving 12 bits for host addressing. This produces 4,096 total IP addresses and 4,094 usable hosts — making it one of the largest subnet sizes in common use. It is particularly popular in AWS for EKS node group subnets, where pod-level IP allocation demands large address pools.
Calculate any /20 subnet instantly →| Property | Value |
|---|---|
| Prefix length | /20 |
| Subnet mask | 255.255.240.0 |
| Wildcard mask | 0.0.15.255 |
| Total addresses | 4,096 |
| Usable host addresses | 4,094 |
| Network address | x.x.0.0 (third octet must be multiple of 16: .0, .16, .32, …) |
| Broadcast address | x.x.15.255 |
| Binary mask | 11111111.11111111.11110000.00000000 |
| AWS usable hosts | 4,091 (AWS reserves 5 IPs per subnet) |
AWS EKS with the VPC CNI plugin assigns a real VPC IP address to every pod. A cluster with 50 nodes running 20 pods each needs 1,000 IPs just for pods, plus node IPs, load balancer IPs, and headroom. A /20 provides 4,091 usable IPs in AWS — enough for large, growing clusters without IP exhaustion.
A /20 subnet's third octet must start on a multiple of 16. Within a 10.0.0.0/16 VPC:
| Subnet | Range | AWS Usable |
|---|---|---|
| 10.0.0.0/20 | 10.0.0.0 – 10.0.15.255 | 4,091 |
| 10.0.16.0/20 | 10.0.16.0 – 10.0.31.255 | 4,091 |
| 10.0.32.0/20 | 10.0.32.0 – 10.0.47.255 | 4,091 |
| 10.0.48.0/20 | 10.0.48.0 – 10.0.63.255 | 4,091 |
| Parent Block | Number of /20s |
|---|---|
| /16 | 16 |
| /8 | 4,096 |
| /24 | 0.0625 (a /20 is 16× larger than a /24) |
| CIDR | Subnet Mask | Total IPs | Usable Hosts | AWS Usable | Typical Use |
|---|---|---|---|---|---|
| /16 | 255.255.0.0 | 65,536 | 65,534 | 65,531 | VPC root CIDR |
| /18 | 255.255.192.0 | 16,384 | 16,382 | 16,379 | Large regional block |
| /20 | 255.255.240.0 | 4,096 | 4,094 | 4,091 | EKS node groups, large AZ subnets |
| /22 | 255.255.252.0 | 1,024 | 1,022 | 1,019 | Large office VLAN |
| /24 | 255.255.255.0 | 256 | 254 | 251 | Standard workload subnet |