Subnet Calculator

About Subnet calculator

Subnet Calculator works out the network address, broadcast address, first and last usable host, address count, netmask and wildcard mask for any IPv4 or IPv6 CIDR block, then splits the block into smaller equal subnets. It is for network engineers, students and anyone planning address space for a VPC, VLAN or office.

How to use it

1. Type a block into the CIDR block field in address/prefix form, for example 192.168.1.0/24. It starts with that value filled in.
2. Or press one of the Try buttons for 192.168.1.0/24, 10.0.0.0/8, 172.16.5.130/26 or 2001:db8::/48.
3. Read the Subnet card, which updates as you type, and press Copy to put the details on your clipboard.
4. In Split into subnets, pick a longer prefix from the New prefix length list to see the resulting subnets in a table.

A /26 taken from the middle of a range

172.16.5.130/26

Network 172.16.5.128, broadcast 172.16.5.191, first host 172.16.5.129, last host 172.16.5.190, 64 addresses, 62 usable, netmask 255.255.255.192, wildcard 0.0.0.63, private range.

When to use it

• Carving a /24 into four /26 subnets, one for each department in an office.
• Checking the exact host range before you write an access control list.
• Converting between prefix length, netmask and wildcard mask for a router configuration.
• Sizing an IPv6 allocation, such as how many /64 networks fit inside a /48.

Questions about Subnet calculator

Do I need to enter the network address itself?

No. Any address inside the block works, and the result is normalized. Entering 172.16.5.130/26 reports the network as 172.16.5.128/26.

Why does a /31 show no broadcast address?

RFC 3021 allows both addresses of a /31 on a point-to-point link, so none is reserved. A /32 is a single host route, and the usable count is one.

Why are IPv6 prefix choices in steps of four?

IPv6 is conventionally divided on nibble boundaries, so the split list moves in steps of 4 bits. Offering all 128 lengths would make the list unusable. IPv6 also has no broadcast address or wildcard mask.

Is there a limit on splits?

The table shows at most 256 subnets, and tells you the true total when there are more. Counts are computed with big integers, so an IPv6 block never overflows.

Is anything sent off my device?

No. All of the arithmetic runs in your browser as you type, and the addresses you enter are never transmitted.

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