IP Subnet Calculator

How to use the IP subnet calculator

1. Choose IPv4 subnet details, splitting a network, or IPv6.
2. Type the address. For IPv4, give the prefix length (such as 24) or the mask (such as 255.255.255.0), or type both together as CIDR, like 10.0.0.0/8.
3. To split a network, enter how many subnets you need. The calculator rounds up to the next power of 2 and lists every subnet.
4. For IPv6, enter the address in any valid form and the prefix length, usually 64 for a LAN or 48 or 56 for a site.

Subnet formulas

An IPv4 address is a 32-bit number. The prefix length says how many of the leading bits name the network; the remaining bits number the hosts inside it. A bitwise AND with the mask clears the host bits and gives the network address, and setting all the host bits to 1 gives the broadcast address. Those two are normally reserved, so a subnet with h host bits has 2^h − 2 usable addresses. There are two exceptions: RFC 3021 lets both addresses of a /31 be used on point-to-point links, and a /32 names a single host. The wildcard mask, used by Cisco ACLs and OSPF, is the mask with every bit flipped.

Worked example: 192.168.10.77/26

A /26 mask is 255.255.255.192: 26 ones, then 6 zeros. In binary the address is 11000000.10101000.00001010.01 | 001101, where the bar marks the end of the network bits. Clearing the 6 host bits gives the network 192.168.10.64, and setting them gives the broadcast 192.168.10.127. Hosts run from 192.168.10.65 to 192.168.10.126: 2⁶ − 2 = 62 usable addresses. The wildcard mask is 0.0.0.63.

Splitting 10.0.0.0/24 into 4 equal subnets borrows 2 bits, giving four /26 networks: 10.0.0.0, 10.0.0.64, 10.0.0.128, 10.0.0.192, each with 62 usable hosts.

CIDR and subnet mask table

Special IPv4 address ranges

The calculator labels each address using the IANA special-purpose registry. Everything else is public, globally routable address space.

IPv6 prefixes

IPv6 addresses are 128 bits, written as eight groups of four hex digits. RFC 5952 sets one canonical short form: lowercase, no leading zeros, and the longest run of zero groups replaced by ::. For example 2001:0db8:abcd:0012:0000:0000:0000:0001 is written 2001:db8:abcd:12::1. Its /64 network is 2001:db8:abcd:12::/64, running to 2001:db8:abcd:12:ffff:ffff:ffff:ffff: 18,446,744,073,709,551,616 addresses. Because the numbers are this large, the calculator works with exact 128-bit integers rather than floating point.

Tips

• Plan for growth. A /24 gives 254 hosts; if a segment might pass 100 devices, don't size it at /25.
• Private ranges for internal networks. Use 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16 (RFC 1918). 100.64.0.0/10 is reserved for carrier-grade NAT, not for home or office LANs.
• Documentation examples. Use 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 and 2001:db8::/32 in guides, so examples never point at a real network.

Planning how fast data moves across the network? The bandwidth calculator works out download times, and the binary calculator and hex calculator help with the bit arithmetic.

Sources

• RFC 4632 (CIDR), RFC 3021 (31-bit prefixes on point-to-point links), RFC 1918 (private addresses), RFC 6598 (shared address space), RFC 6890 and the IANA IPv4 and IPv6 Special-Purpose Address Registries.
• RFC 4291 (IPv6 addressing architecture), RFC 5952 (IPv6 text representation), RFC 4193 (unique local addresses), RFC 3849 (IPv6 documentation prefix).