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MtrXTools

Subnet calculator

Work out network and broadcast addresses, usable host ranges, masks and subnet splits for any IPv4 or IPv6 CIDR block. Runs in your browser.

What the calculator shows

Enter an address with a prefix length, such as 10.20.0.0/22 or 2001:db8::/48. You get the network address, the first and last usable host, the broadcast address (IPv4), the netmask, the number of addresses, and how the block splits into smaller subnets.

Quick reference

  • /24: 256 addresses, 254 usable hosts, netmask 255.255.255.0.
  • /30: 4 addresses, 2 usable, the classic point-to-point link (/31 gives 2 usable on links that support it).
  • /64: the standard IPv6 subnet size; SLAAC needs exactly 64 bits for the host part.

Questions

Why are two addresses not usable in IPv4?
The first address identifies the network and the last is the broadcast address. IPv6 has no broadcast, so every address in the block is usable.
Need to merge or plan many ranges?
Use the Network planner to summarise CIDRs, check which range contains an address, or split an IPv6 prefix across sites.
How do I pick a subnet size?
Count the hosts you need, add room to grow, and round up to the next power of two, then add the two reserved IPv4 addresses. Cloud providers reserve a few more per subnet (AWS keeps five), so check your platform.

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