/20 subnet: mask 255.255.240.0, 4,094 usable hosts

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Updated Oct 8, 2026

Examples

IP address / CIDR / netmask

IPv4 or IPv6, with or without a prefix — 10.0.0.0/8, 2001:db8::/48, or a dotted netmask like 192.168.1.0 255.255.255.0. A bare address means one host (/32 or /128).

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fig. 10 — subnet-calculator · networking /20 — 4094 usable hosts

Result

IPv410.0.0.0/20

Usable hosts4094

Usable range10.0.0.1 – 10.0.15.254

Total addresses4096

Addressing

Network address: 10.0.0.0
Broadcast address: 10.0.15.255
Usable host range: 10.0.0.1 – 10.0.15.254

Masks

Netmask: 255.255.240.0
Wildcard maskInverse of the netmask — the match form Cisco ACLs and OSPF expect.: 0.0.15.255
Netmask (binary)The mask bit by bit — the 1s are the network part.: 11111111.11111111.11110000.00000000

Details

Address type: Private (RFC 1918)
Network (integer)The network address as a single 32-bit number, as scripts and databases store it.: 167 772 160

Check against CIDRs Split this subnet Convert this address

A /20 gives 4,096 addresses and 4,094 usable hosts under netmask 255.255.240.0. It is sixteen /24s in one block, a common size for cloud subnets that host containers or autoscaling groups.

Reading the /20 mask

Twenty network bits fill the first two octets and four bits of the third, so the mask is 255.255.240.0 and the wildcard is 0.0.15.255. The third octet therefore steps in blocks of 16: a /20 can start at .0, .16, .32 and so on, never at .8.

The host count is 2^12 = 4,096. With network and broadcast removed, 4,094 addresses are usable. In 10.0.0.0/20 that is 10.0.0.1 through 10.0.15.254, broadcast 10.0.15.255.

Why cloud teams pick /20

AWS default VPCs create one /20 per availability zone inside 172.31.0.0/16, which made the size familiar. After the five AWS-reserved addresses a /20 subnet leaves 4,091 assignable IPs, enough for EKS or ECS workloads where every pod or task takes a VPC address.

It is also a comfortable unit for IP address management: big enough that a busy subnet does not run dry during a scale-out, small enough that a /16 still yields sixteen of them.

Neighbouring sizes

Halve a /20 and you get two /21s; quarter it for four /22s. Doubling goes the other way: two aligned /20s make a /19. If a /20 is more than a workload needs, a /22 (about a thousand hosts) is the usual next step down.

Watch alignment when you split by hand. 10.0.8.0/20 is not a valid network address; the calculator will report it as a host inside 10.0.0.0/20.

Configuring and checking

The gateway for 10.0.0.0/20 is conventionally the first usable address, 10.0.0.1, though nothing enforces it. Hosts must carry the full 255.255.240.0 mask; a host left on 255.255.255.0 would treat everything outside its own /24 as remote and push same-subnet traffic through the router.

Before carving a VPC, list the /20s you need per availability zone and per tier. Three zones times public, private and data tiers is nine /20s, which still leaves seven spare in a /16 for later services.

FAQ

Questions, answered.

Tap a question to expand the answer.

How many /24s are in a /20?

Sixteen. A /20 spans sixteen consecutive values of the third octet, starting on a multiple of 16.

How many usable IPs does a /20 have on AWS?

4,091. AWS reserves the first four addresses and the last one in every subnet, so 4,096 minus 5.

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