DevToolsmith

CIDR and Subnet Calculator

Enter an IP address with a prefix (like 10.0.0.0/16) or a subnet mask to get the network, broadcast, mask, usable host range and host count. Works for IPv4 and IPv6. Check if an IP is in range, or split a block into smaller subnets.

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Prefix:/24
Network
192.168.10.0/24
Subnet mask
255.255.255.0
Wildcard mask
0.0.0.255
Usable host range
192.168.10.1 – 192.168.10.254
Broadcast
192.168.10.255
Total addresses
256
Usable hosts
254
Address type
Private (RFC 1918)
Binary address
11000000.10101000.00001010.00000000
Binary mask
11111111.11111111.11111111.00000000
Integer / hex
3232238080 / 0xC0A80A00

Is an IP in this range?

✓ 192.168.10.77 is inside 192.168.10.0/24

Split into subnets

4 subnets
68 chars · 68 bytes · 4 lines

IPv4 CIDR cheat sheet

CIDRSubnet maskAddressesUsable hosts
/8 255.0.0.0 16,777,216 16,777,214
/12 255.240.0.0 1,048,576 1,048,574
/16 255.255.0.0 65,536 65,534
/20 255.255.240.0 4,096 4,094
/22 255.255.252.0 1,024 1,022
/23 255.255.254.0 512 510
/24 255.255.255.0 256 254
/25 255.255.255.128 128 126
/26 255.255.255.192 64 62
/27 255.255.255.224 32 30
/28 255.255.255.240 16 14
/29 255.255.255.248 8 6
/30 255.255.255.252 4 2
/31 255.255.255.254 2 2
/32 255.255.255.255 1 1

How CIDR notation works

An IPv4 address is 32 bits. CIDR (Classless Inter-Domain Routing) notation writes an address plus how many of those bits belong to the network. For 192.168.10.77/26:

  • The mask has 26 one-bits: 255.255.255.192.
  • ANDing the address with the mask gives the network address: 192.168.10.64.
  • Setting the remaining 6 host bits to 1 gives the broadcast address: 192.168.10.127.
  • Everything in between is usable: 192.168.10.65 to 192.168.10.126, which is 62 hosts.

Planning subnets for a cloud VPC

  • Start big: give the VPC a large private block such as 10.0.0.0/16 (65,536 addresses). You can't easily grow it later.
  • Split evenly: carve it into /20 or /24 subnets per availability zone and tier (public, private, database) with the Split into subnets tool above.
  • Avoid overlaps: don't reuse ranges that you may need to peer with other VPCs, on-premises networks or a VPN. 192.168.0.0/16 clashes with many home routers.
  • Kubernetes needs room: pod and service CIDRs must not overlap the node network, and CNIs that give each pod a VPC IP use up addresses quickly.

IPv6 prefixes

IPv6 addresses are 128 bits. Providers typically assign a /48 or /56 to a site, and each LAN gets a /64. That's 18 quintillion addresses per subnet, so you count subnets rather than hosts. The calculator shows compressed and fully expanded forms and recognizes link-local, unique local, documentation and multicast ranges.

Frequently asked questions

What does /24 mean in an IP address?

The number after the slash is the prefix length: how many leading bits identify the network. /24 means the first 24 bits (the first three numbers in 192.168.1.0/24) are the network, leaving 8 bits for hosts. That is 256 addresses, 254 of them usable for hosts.

How many usable hosts are in a subnet?

Usable hosts = 2^(32 − prefix) − 2, because the first address is the network address and the last is the broadcast address. A /24 has 254 usable hosts, a /26 has 62. The exceptions are /31, which has 2 usable addresses for point-to-point links (RFC 3021), and /32, a single host.

Why does AWS say my /24 subnet only has 251 IPs?

AWS reserves five addresses in every VPC subnet: the network address, the next three (for the VPC router, DNS and future use), and the last address. Azure also reserves five. Plan cloud subnets with that overhead in mind.

What are the private IP address ranges?

RFC 1918 defines 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0–172.31.255.255) and 192.168.0.0/16 for private networks. 100.64.0.0/10 is reserved for carrier-grade NAT. For IPv6, the private equivalent is unique local addresses in fc00::/7, usually fd00::/8.

What is a wildcard mask?

A wildcard mask is the inverse of the subnet mask: 0.0.0.255 for a /24. Cisco ACLs and OSPF configuration use it to match address ranges.