/21 subnet: mask 255.255.248.0, 2,046 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 /21 — 2046 usable hosts

Result

IPv410.0.0.0/21

Usable hosts2046

Usable range10.0.0.1 – 10.0.7.254

Total addresses2048

Addressing

Network address: 10.0.0.0
Broadcast address: 10.0.7.255
Usable host range: 10.0.0.1 – 10.0.7.254

Masks

Netmask: 255.255.248.0
Wildcard maskInverse of the netmask — the match form Cisco ACLs and OSPF expect.: 0.0.7.255
Netmask (binary)The mask bit by bit — the 1s are the network part.: 11111111.11111111.11111000.00000000

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 /21 uses netmask 255.255.248.0 and holds 2,048 addresses, 2,046 of them usable. It equals eight /24s and fits large Wi-Fi or campus segments.

The /21 at bit level

Twenty-one ones put five network bits in the third octet, so the mask is 255.255.248.0 and the wildcard 0.0.7.255. Valid /21 networks start at third-octet values that are multiples of 8: .0, .8, .16, .24.

The block holds 2^11 = 2,048 addresses. Drop the network and broadcast addresses and 2,046 are left; for 10.0.0.0/21 the usable range is 10.0.0.1 to 10.0.7.254 with broadcast 10.0.7.255.

Typical uses

Guest and corporate Wi-Fi often lands on a /21: devices roam, leases pile up, and phones with randomised MAC addresses can hold several leases each. Two thousand addresses absorbs that churn without shortening the DHCP lease time to minutes.

In a cloud VPC a /21 leaves 2,043 assignable addresses after the five AWS or Azure reserve, a middle ground between a /22 and a /20 for container subnets.

Pitfalls

A large flat segment has a large broadcast domain. If you put two thousand chatty hosts on one VLAN, ARP, mDNS and DHCP traffic grows with them; on Wi-Fi, where broadcasts go out at the lowest data rate, that costs airtime. Client isolation and multicast filtering help.

When splitting, a /21 becomes two /22s or eight /24s. Its neighbour above 10.0.0.0/21 is 10.0.8.0/21, and the pair aggregate to a /20.

Planning DHCP on a /21

A DHCP scope on 10.0.0.0/21 can hand out almost the whole block, 10.0.0.1 to 10.0.7.254, minus whatever you reserve for the gateway, controllers and printers. Keep the exclusions at one end so the pool stays one contiguous span; scattered exclusions are easy to forget when the subnet is later resized.

Lease time matters more on a big guest network than the size of the block. A long lease on a busy event network can exhaust even two thousand addresses, while a lease of an hour or two returns addresses as visitors leave.

FAQ

Questions, answered.

Tap a question to expand the answer.

Is 10.0.4.0/21 a valid network?

No. A /21 must start on a third-octet multiple of 8, so 10.0.4.0 is a host inside 10.0.0.0/21.

How many /24s does a /21 contain?

Eight, covering third-octet values from the block start through start + 7.

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