IP Subnet & VLSM Calculator

MAG212 CYBER · Tools

IP subnet & VLSM calculator.

Type any IPv4 address with a prefix, mask or wildcard. Get the network, broadcast, host range, binary view and ready-to-paste Cisco commands — then carve a block into right-sized subnets.

Subnet calculatorVLSM plannerQuick reference

Subnet calculator

Accepts 10.20.20.77/26, 10.20.20.77 255.255.255.192 or a Cisco wildcard such as 10.20.20.0 0.0.0.63.

Try:

Enter an IPv4 address followed by /prefix (0–32), a subnet mask, or a wildcard mask.

Binary view — network bits · host bits

Cisco IOS / IOS XE snippets



The interface example uses the address you typed when it is a usable host, otherwise the first usable address. OSPF and ACLs use wildcard masks — learn more in the OSPF cheat sheet.




VLSM planner
Give a parent block and one line per subnet (name hosts). Subnets are allocated largest-first so every block lands on a valid boundary with no overlap.











Quick reference
PrefixSubnet maskWildcardUsable hosts
/8255.0.0.00.255.255.25516,777,214
/12255.240.0.00.15.255.2551,048,574
/16255.255.0.00.0.255.25565,534
/17255.255.128.00.0.127.25532,766
/18255.255.192.00.0.63.25516,382
/19255.255.224.00.0.31.2558,190
/20255.255.240.00.0.15.2554,094
/21255.255.248.00.0.7.2552,046
/22255.255.252.00.0.3.2551,022
/23255.255.254.00.0.1.255510
/24255.255.255.00.0.0.255254
/25255.255.255.1280.0.0.127126
/26255.255.255.1920.0.0.6362
/27255.255.255.2240.0.0.3130
/28255.255.255.2400.0.0.1514
/29255.255.255.2480.0.0.76
/30255.255.255.2520.0.0.32
/31255.255.255.2540.0.0.12 (point-to-point, RFC 3021)
/32255.255.255.2550.0.0.01 (host route)


Remember: usable hosts = 2(32 − prefix) − 2, because the network and broadcast addresses can’t be assigned. Exceptions: /31 gives two usable addresses on point-to-point links (RFC 3021) and /32 identifies a single host, such as a loopback.


🔒 Every calculation runs in your browser. Nothing you type is sent or stored.


⚖️ For education and use on systems you own or are authorised to test. Results are automated guidance, not a professional audit. See our terms of use and privacy policy.


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