Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation describes a network as an IP address plus a prefix length, for example 192.168.1.0/24. The prefix says how many leading bits identify the network; the remaining bits number the hosts inside it. A /24 leaves 8 host bits (256 addresses), a /16 leaves 16 (65,536 addresses). CIDR replaced the old class A/B/C system in 1993 and is how every modern router, firewall and cloud VPC describes address space.

A /24 has 8 host bits, giving 256 addresses, of which 254 are usable. Two are reserved in every ordinary subnet: the lowest address is the network address (identifies the subnet itself) and the highest is the broadcast address (reaches every host on the subnet). The same minus-2 rule applies from /1 through /30 — only /31 point-to-point links and /32 single hosts are exceptions.

The network address is the lowest address in a subnet (all host bits set to 0) — it names the subnet in routing tables and can't be assigned to a device. The broadcast address is the highest address (all host bits set to 1) — packets sent to it are delivered to every host in the subnet. Everything between the two is the usable host range.

A /31 contains just two addresses, with no room for separate network and broadcast addresses — RFC 3021 allows both to be used on point-to-point links between routers, which is why modern networks prefer /31 over the older /30 for router interconnects. A /32 is a single host address, the form you'll see in firewall rules, host routes and loopback interfaces.

A wildcard mask is the bitwise inverse of the subnet mask: a /24 mask of 255.255.255.0 becomes the wildcard 0.0.0.255. Cisco IOS access-control lists and OSPF/EIGRP network statements use wildcard masks instead of subnet masks, so network engineers constantly convert between the two — this calculator shows both for every prefix.

RFC 1918 reserves three ranges for private networks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Addresses in these ranges are never routed on the public internet. Related special ranges include 100.64.0.0/10 (carrier-grade NAT), 169.254.0.0/16 (link-local / APIPA) and 127.0.0.0/8 (loopback). The calculator badges every address you enter with its classification automatically.

Cloud providers reserve extra addresses in every subnet beyond the standard two. AWS reserves five per subnet (network, VPC router, DNS, one for future use, and broadcast), so a /24 gives 251 usable addresses instead of 254; Azure also reserves five, GCP four. This calculator shows standard IPv4 math — subtract your provider's extra reservations when sizing cloud subnets.

Each extra prefix bit doubles the number of subnets and halves their size: a /24 splits into two /25s of 126 hosts, four /26s of 62 hosts, or sixteen /28s of 14 hosts. Switch this calculator to Advanced mode and use the subnet splitter — pick a target prefix and it lists every resulting subnet with its network address, host range and broadcast, ready to paste into an IP plan.

This page focuses on IPv4, where subnet math (usable hosts, broadcast addresses, minus-2 rules) has its own quirks. For IPv6 we have a dedicated IPv6 subnet calculator with 128-bit math: address compression and expansion, prefix ranges, exact address counts, ip6.arpa reverse DNS and /48-to-/64 planning. IPv6 subnetting genuinely differs — there are no broadcast addresses and virtually every LAN is a /64.