Subnet Calculator – IP Subnet Mask & CIDR Calculator

How to use the subnet calculator

• Subnet info: type an IPv4 address with a prefix (192.168.1.10/24), a mask (192.168.1.10 255.255.255.0), or a CIDR block (192.168.1.0/24) in one field, or split the prefix/mask into the second field. You get the network, broadcast, usable host range, subnet and wildcard masks, IP class, address type (private/public/reserved), binary and hex views, and the in-addr.arpa reverse-DNS zone.
• Split subnets: enter a base network, then split it evenly into a chosen number of subnets, or by the number of hosts each subnet needs (VLSM) — paste one label,hosts line per requirement and they are packed largest-first with no overlap.
• Range ↔ CIDR: enter a start and end address to get the smallest list of CIDR blocks covering that range. To go the other way — CIDR block to IP range — use the Subnet info tab.
• IPv6: enter an IPv6 address and prefix length to get the network, first and last address, and the exact address count.

What is a subnet mask?

An IPv4 address is 32 bits split into a network part and a host part. A subnet mask marks that split: it is 32 bits too, with a 1 for every network bit and a 0 for every host bit, written in the same dotted-quad form as an address. 255.255.255.0 means the first 24 bits are the network and the last 8 are hosts — the same information as writing /24 after the address.

Applying the mask (address AND mask) to any address in the subnet gives the same network address. For 192.168.1.10/24:

Because 192.168.1.10 has non-zero bits in the host part, it is a host address inside 192.168.1.0/24, not the network address itself — the calculator flags this whenever it happens.

How many hosts fit in each prefix? A non-octet example

Prefixes that don't land on an 8-bit boundary work the same way. 192.168.1.40/27 uses mask 255.255.255.224 (only the last octet is split): the block runs from 192.168.1.32 to 192.168.1.63, giving 30 usable hosts, 192.168.1.33 to 192.168.1.62. A smaller /30 such as 10.0.0.4/30 fits exactly 2 hosts — the classic size for a point-to-point link when a full /31 isn't used.

What is CIDR notation?

CIDR (Classless Inter-Domain Routing, RFC 4632) replaced the rigid class-based system (Class A = /8, B = /16, C = /24) with an address plus an explicit prefix length, e.g. 192.168.1.10/24. Any prefix length from /0 (the whole internet) to /32 (one address) is valid, which is what lets internet service providers and cloud platforms hand out blocks sized to what a customer actually needs instead of the nearest class boundary.

Splitting a network into subnets (VLSM)

Splitting 192.168.1.0/24 into 4 equal subnets moves the prefix from /24 to /26 (2 extra bits = 4 blocks), each with 62 usable hosts:

Real networks rarely need equal-sized subnets. Variable Length Subnet Masking (VLSM) gives each department or link only the block it needs: a 100-host office gets a /25, a 50-host one a /26, and a 2-host router link a /30, packed into the base network without gaps or overlap. The "Split subnets" tab does this automatically — list each requirement as label,hosts and it sorts largest-first and allocates each the smallest block that fits.

CIDR to IP range and IP range to CIDR

Converting a CIDR block to a range is direct — the calculator gives it on the Subnet info tab. The reverse is not always one block: a range only maps to a single CIDR block when it starts on a power-of-two boundary and its length is itself a power of two. 192.168.1.5–192.168.1.20 needs 5 blocks:

Private vs public IP addresses

RFC 1918 sets aside three ranges for private networks never routed on the public internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. A few other ranges have special jobs: 127.0.0.0/8 is loopback, 169.254.0.0/16 is link-local (self-assigned 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 documentation-only ranges, like the examples on this page. The calculator flags all of these under "Address type".

IPv6 subnetting

IPv6 addresses are 128 bits, written as eight hex groups with :: allowed once to compress a run of zero groups. A home allocation is typically a /64 — 2⁶⁴ addresses, more than every IPv4 address ever issued, in one subnet — while an ISP customer often gets a /56 or /48 to split into many /64s. The IPv6 tab gives the network, first/last address and the exact address count as a full integer (JavaScript numbers can't hold it exactly, so it uses arbitrary-precision arithmetic).

Subnet cheat sheet: every prefix /0 to /32

Every prefix length, its mask, wildcard mask, and address/host counts — generated by the same engine as the calculator above, so it always matches:

Reverse DNS and the in-addr.arpa zone

Reverse DNS (an IP address to a hostname, via a PTR record) is served from the in-addr.arpa zone, with the address octets reversed: 192.168.1.0/24 is delegated in 1.168.192.in-addr.arpa, and the whole 10.0.0.0/8 block is 10.in-addr.arpa. Prefixes off an octet boundary (like /27) are delegated within their containing /24 (RFC 2317), so the calculator shows that whole-octet zone for those.

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Frequently asked questions