About this tool
CIDR notation expresses an IPv4 network as an address plus prefix length, such as 172.16.24.0/21. The prefix tells you how many high-order bits belong to the network. From that, the address range, mask, broadcast and capacity can be derived.
This calculator is useful when a ticket, route table, firewall rule or IPAM entry gives you only a CIDR block and you need to see the exact first and last addresses it covers.
How to use it
- Enter a valid IPv4 CIDR block.
- Calculate the network range.
- Compare the returned boundaries with the route, ACL, DHCP pool or customer allocation you are reviewing.
Example: 172.16.24.0/21
A /21 spans eight /24-sized blocks. The range runs from 172.16.24.0 through 172.16.31.255, with a subnet mask of 255.255.248.0.
Engineering notes
- A CIDR block must begin on a boundary that matches its prefix when used as a network statement. Host addresses can still be entered in many tools, but the calculated network will snap to the correct boundary.
- CIDR is classless; the old Class A, B and C boundaries do not determine modern subnet size.
Frequently asked questions
How many addresses are in a /21?
A /21 contains 2^(32-21), or 2,048 total IPv4 addresses.
Is the last address always broadcast?
In traditional IPv4 subnetting, yes. Special cases such as /31 point-to-point links are handled differently by supporting devices.
Can a CIDR range cross an octet boundary?
Yes. Prefixes do not have to end on /8, /16 or /24 boundaries.