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CIDR Calculator

Interactive CIDR calculator with live binary visualization. Adjust each octet and prefix to see the netmask, base IP, broadcast IP, address count, and usable host range instantly.

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/
/0/16/32
10101100
00011111
00000000
00000000
Network bits (16) Host bits (16)
255.255.0.0
Netmask
172.31.0.0
CIDR Base IP
172.31.255.255
Broadcast IP
65,536
Address Count
172.31.0.1
First Usable IP
172.31.255.254
Last Usable IP
Wildcard mask: 0.0.255.255 IP type: Private (RFC 1918)

How CIDR Notation Works

CIDR (Classless Inter-Domain Routing) describes a block of IP addresses as a base address plus a prefix length: 10.42.8.0/22. The prefix length says how many of the 32 bits in an IPv4 address belong to the network. The visualization above colors those network bits; everything after them is host space you can assign to devices.

Change any octet or drag the prefix slider and the block updates live — the base IP snaps to the true network boundary, and the count, broadcast, and usable range recalculate.

Formula

The size of a CIDR block depends only on the prefix length p:

  • Total addresses = 2(32 − p)
  • Usable hosts = 2(32 − p) − 2 (for /30 and larger blocks; /31 has 2 usable per RFC 3021, /32 is a single host)
  • Base IP = address AND netmask (host bits zeroed)
  • Broadcast IP = base IP OR wildcard mask (host bits set to 1)

Worked example — 10.42.8.0/22: 32 − 22 = 10 host bits, so 210 = 1,024 addresses. The block runs from 10.42.8.0 (base) to 10.42.11.255 (broadcast), with 10.42.8.1 – 10.42.11.254 usable — 1,022 hosts.

CIDR Block Reference Table

CIDRNetmaskAddressesTypical Use
/8 255.0.0.0 16,777,216 Largest private range (10/8)
/12 255.240.0.0 1,048,576 Private range (172.16/12)
/16 255.255.0.0 65,536 Large VPC or campus network
/19 255.255.224.0 8,192 Large ISP customer allocation
/20 255.255.240.0 4,096 Cloud VPC subnet tier
/21 255.255.248.0 2,048 Mid-size site network
/22 255.255.252.0 1,024 Office or school network
/23 255.255.254.0 512 Two joined /24 networks
/24 255.255.255.0 256 Standard LAN subnet
/25 255.255.255.128 128 Half a /24
/26 255.255.255.192 64 Small department subnet
/27 255.255.255.224 32 Small office or VLAN
/28 255.255.255.240 16 Server or DMZ segment
/29 255.255.255.248 8 Tiny segment (6 hosts)
/30 255.255.255.252 4 Traditional point-to-point link
/32 255.255.255.255 1 Single host route

Understanding the Terms

Prefix Length

The number after the slash — how many leading bits identify the network. A smaller prefix means a bigger block: each step down doubles the address count.

Netmask

The prefix expressed as dotted decimal: /20 becomes 255.255.240.0. Operating systems and routers use it to decide whether a destination is on the local subnet or must go through a gateway.

Wildcard Mask

The inverse of the netmask, used in Cisco access lists and OSPF configuration. For /20 the wildcard is 0.0.15.255.

Base IP (Network Address)

The first address of the block, host bits all zero. It identifies the network in routing tables and cannot be assigned to a device on standard subnets.

Broadcast IP

The last address of the block, host bits all one. Traffic sent here reaches every host on the subnet.

CIDR in Cloud Networking

CIDR planning is a day-one decision in every cloud deployment because VPC blocks are hard to change later:

  • AWS: VPCs accept /16 to /28. The default VPC is 172.31.0.0/16 with /20 subnets per availability zone. Peered VPCs must not overlap.
  • Azure: Virtual networks commonly use a /16 with /24 subnets; Azure reserves 5 addresses per subnet instead of 2, so a /24 yields 251 usable.
  • Google Cloud: Subnets are regional; a /20 per region from a 10.x plan is a common pattern.
  • Kubernetes: Pod and service ranges each need their own non-overlapping block — a /16 pod range supports roughly 65k pods.

Rule of thumb: allocate at least four times the addresses you think you need, and keep every environment (prod, staging, VPN, office) on non-overlapping blocks so they can be connected later.

Related Tools

  • IP Subnet Calculator — full IPv4/IPv6 subnet details including IP class, wildcard mask, and an IPv6 prefix breakdown
  • Hex Calculator — convert between binary, decimal, and hexadecimal, with bitwise operations

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Frequently Asked Questions

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation writes a network as an IP address followed by a slash and a prefix length, like 10.42.8.0/22. The prefix length is the number of leading bits that identify the network; the remaining bits identify hosts within it. A /22 has 22 network bits and 10 host bits, so it spans 2^10 = 1,024 addresses.

How do I calculate the number of IP addresses in a CIDR block?

Raise 2 to the power of (32 minus the prefix length). A /16 contains 2^16 = 65,536 addresses, a /24 contains 2^8 = 256, and a /28 contains 2^4 = 16. Subtract 2 (network and broadcast addresses) to get usable host addresses on a normal subnet.

What is the difference between the CIDR base IP and the broadcast IP?

The base IP (network address) is the first address in the block, with all host bits set to 0 — it names the network itself. The broadcast IP is the last address, with all host bits set to 1 — packets sent to it reach every host on the subnet. Neither is assignable to a device on subnets of /30 or larger blocks.

What does a netmask like 255.255.0.0 mean?

A netmask is the prefix length written as a dotted decimal number: the first N bits are 1 and the rest are 0. For /16 that is 16 ones followed by 16 zeros, which reads as 255.255.0.0. The netmask ANDed with any IP in the block yields the network base address.

Why does this calculator show a different base IP than the address I typed?

If you enter an address whose host bits are not all zero — say 192.168.5.77/24 — the calculator masks it down to the true network base, 192.168.5.0. Routers do the same thing: the host portion is ignored when identifying the network a packet belongs to.

What is a /31 subnet used for?

RFC 3021 allows /31 subnets on point-to-point links between two routers. With only 2 addresses and no other hosts, a dedicated network and broadcast address are unnecessary, so both addresses are usable. This halves address consumption on router-to-router links compared to the traditional /30.

Which CIDR blocks are private?

RFC 1918 reserves three ranges for private networks: 10.0.0.0/8 (16.7 million addresses), 172.16.0.0/12 (1 million addresses), and 192.168.0.0/16 (65,536 addresses). These are free to use internally but are not routable on the public internet.

What CIDR block should I use for a cloud VPC?

AWS, Azure, and Google Cloud all recommend an RFC 1918 block. The AWS default VPC uses 172.31.0.0/16; a common custom choice is a /16 from 10.0.0.0/8, subdivided into /20 or /24 subnets per availability zone. Pick a block that does not overlap any network you may later peer or VPN with.

What is a wildcard mask?

A wildcard mask is the bitwise inverse of the netmask — 0.0.0.255 for a /24. Cisco ACLs and OSPF network statements use wildcard masks to express which address bits must match: 0 bits must match exactly, 1 bits are ignored.

Can a CIDR prefix be larger than /32 or negative?

No. IPv4 addresses are 32 bits, so valid prefixes run from /0 (the entire IPv4 space, 4.29 billion addresses) to /32 (a single host). IPv6 uses the same notation with prefixes up to /128.