Subnet calculator
CIDR, netmask, hosts.

What CIDR notation means

An IPv4 address is 32 bits, written as four decimal numbers from 0 to 255. Classless Inter-Domain Routing (CIDR, RFC 4632) splits those bits into a network part and a host part, and the number after the slash says how many bits belong to the network. In 192.168.1.10/24 the first 24 bits (192.168.1) identify the network and the last 8 bits number the hosts inside it.

Before CIDR, networks came in fixed classes: A (/8), B (/16) and C (/24). Classes wasted space, because an organisation needing 300 addresses had to take a whole class B of 65,536. CIDR allows any prefix length, which is why routing tables, firewall rules and cloud consoles all speak in prefixes today. The class is still shown in the result because older documentation and some exams use it.

Network, broadcast and usable hosts

A subnet of prefix n holds 2^(32 − n) addresses. The first one, with all host bits set to 0, is the network address that names the subnet in routing tables. The last one, with all host bits set to 1, is the broadcast address that reaches every host on the segment. Neither can be given to a device, so a classic subnet has 2^(32 − n) − 2 usable hosts: 254 in a /24, 62 in a /26, 6 in a /29.

Two prefixes break that rule. A /31 has only two addresses, and RFC 3021 lets routers use both on a point-to-point link because such a link needs no broadcast. Most modern router and firewall software supports it, and it halves the addresses spent on transit links compared with a /30. A /32 is a single address: it describes one host, a loopback interface, an anycast service or a firewall rule for exactly one IP.

Netmask, wildcard mask and prefix length

A netmask writes the prefix as an address: n one-bits followed by zeros. /24 is 255.255.255.0, /20 is 255.255.240.0 and /29 is 255.255.255.248. A valid netmask always has its ones on the left and its zeros on the right; a value like 255.0.255.0 is not a netmask, and the calculator says so. To convert a mask to a prefix by hand, count the one-bits: 255 counts 8, 254 counts 7, 252 counts 6, 248 counts 5, 240 counts 4, 224 counts 3, 192 counts 2 and 128 counts 1. What is a subnet mask? walks through the binary AND and has the full /0 to /32 cheat sheet.

A wildcard mask is the inverse: zeros for the network bits, ones for the host bits, so /24 becomes 0.0.0.255. Cisco access lists and OSPF network statements use it. The converter under the result turns any of the three forms into the other two.

Common subnet sizes

• /24 subnet: mask 255.255.255.0, 256 addresses, 254 usable. The default size for a home or office LAN and a VLAN.
• /29 subnet mask: 255.255.255.248, 8 addresses, 6 usable. A typical block of static public IPs from an ISP; one address usually goes to the provider's gateway.
• /30: 255.255.255.252, 2 usable. The traditional size for a point-to-point link, now often replaced by a /31.
• /16: 255.255.0.0, 65,534 usable. Common as the outer range of a cloud VPC, which is then cut into /20 or /24 subnets.
• /22: 255.255.252.0, 1,022 usable. Large Wi-Fi or campus networks where a /24 runs out.

Private, CGNAT and other special ranges

Some ranges never appear on the public internet. RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 for private networks. 100.64.0.0/10 is shared address space for carrier-grade NAT (RFC 6598): an address from it on your router's WAN side means your provider shares one public IP between many customers. 127.0.0.0/8 is loopback, 169.254.0.0/16 is link-local (what a device assigns itself when DHCP fails), 224.0.0.0/4 is multicast, and 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 are reserved for documentation.

The calculator labels each of these in the result, and private IP address ranges lists the special-purpose blocks with the RFC that defines each. To see who a public address belongs to, use the IP & ASN lookup; to see which addresses a domain points to, use the DNS lookup. For how address blocks are routed between networks, see what BGP is.