Subnet calculator (IPv4 and IPv6)

For IPv4, you get:

• Network address and prefix, e.g. 192.168.1.0/24
• Subnet mask (255.255.255.0) and wildcard mask (0.0.0.255, used in Cisco ACLs and OSPF)
• Broadcast address (192.168.1.255)
• First and last usable host (192.168.1.1 – 192.168.1.254)
• Usable hosts and total addresses
• Address type (private, public, loopback, CGNAT and so on) and the legacy class
• the address and mask in binary, which makes the network/host split visible
• the previous and next subnet of the same size

For IPv6, you get the compressed and expanded forms, network, first and last address, number of addresses and how many /64 subnets fit. Everything is calculated in your browser.

Prefix	Subnet mask	Total addresses	Usable hosts
/8	255.0.0.0	16,777,216	16,777,214
/16	255.255.0.0	65,536	65,534
/20	255.255.240.0	4,096	4,094
/22	255.255.252.0	1,024	1,022
/23	255.255.254.0	512	510
/24	255.255.255.0	256	254
/25	255.255.255.128	128	126
/26	255.255.255.192	64	62
/27	255.255.255.224	32	30
/28	255.255.255.240	16	14
/29	255.255.255.248	8	6
/30	255.255.255.252	4	2
/31	255.255.255.254	2	2 (point-to-point, RFC 3021)
/32	255.255.255.255	1	1 (single host)

The rule: total addresses = 2^(32 − prefix). Subtract 2 for the network and broadcast address to get usable hosts, except for /31 and /32.

Take 192.168.1.10/24:

1. The prefix /24 means the first 24 bits are the network part, giving the mask 255.255.255.0.
2. Network address: the IP AND the mask → 192.168.1.0.
3. The remaining 8 bits are for hosts: 2^8 = 256 addresses.
4. Broadcast: all host bits set to 1 → 192.168.1.255.
5. Usable hosts: 192.168.1.1 to 192.168.1.254, which is 254 hosts.

Now 10.0.0.1 255.255.240.0: the mask equals /20, so the network is 10.0.0.0/20 with 4,096 addresses, usable hosts 10.0.0.1 to 10.0.15.254, and broadcast 10.0.15.255.

Choose a longer prefix under Split into smaller subnets and the calculator lists every resulting subnet with its host range and broadcast address. For example, 192.168.1.0/24 split into /26 gives four subnets of 62 hosts each:

Subnet	Hosts	Broadcast
192.168.1.0/26	.1 – .62	.63
192.168.1.64/26	.65 – .126	.127
192.168.1.128/26	.129 – .190	.191
192.168.1.192/26	.193 – .254	.255

This is the everyday task when you plan VLANs for an office, carve a cloud VPC into public and private subnets, or set up a home lab. Note that cloud providers reserve some addresses in each subnet; AWS, for example, reserves five, so a /24 subnet there has 251 usable addresses.

Range	Purpose
10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16	Private networks (RFC 1918), not routed on the internet
100.64.0.0/10	Carrier-grade NAT (RFC 6598), used by ISPs
127.0.0.0/8	Loopback (localhost)
169.254.0.0/16	Link-local; an address here usually means DHCP failed
224.0.0.0/4	Multicast
192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24	Reserved for documentation and examples

For IPv6, fe80::/10 is link-local, fc00::/7 is unique local (the private-network equivalent), 2001:db8::/32 is for documentation, and 2000::/3 is global unicast. A standard IPv6 LAN is a /64.