IP & subnet tools
Network engineering work involves splitting an address block into subnets, merging a list of CIDR ranges into the minimal set, or decoding the flag byte from a captured TCP packet. This page covers subnet and CIDR math for IPv4 and IPv6, address and MAC formatting, and lookups against live network data when you need to identify who owns an address or test reachability.
The subnet calculator, CIDR set calculator and IP range to CIDR converter all handle address math locally, so use whichever matches your starting point — a netmask, a set of existing CIDRs, or a plain IP range. The ASN lookup and network connectivity test are the two tools that reach out to public BGP data and remote probes respectively; everything else, including the MAC address formatter and TCP flag decoder, works entirely on the values you type in.
8 tools
Calculate and merge CIDR blocks
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Subnet calculator
Determine network boundaries and host counts with CIDR and netmask support
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CIDR set calculator
Merge, split, intersect or exclude IPv4 and IPv6 networks
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IP range to CIDR
Convert inclusive IP ranges to minimal CIDR blocks without address loss
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IP converter
Convert IPv4 and IPv6 between decimal, hex, binary and standard notation
IP & Subnet Tools tools FAQ
The CIDR set calculator takes a list of IPv4 or IPv6 networks and merges, intersects or excludes them as needed, producing the minimal set of blocks covering the same addresses. It's the tool to use before writing a firewall rule or an ACL from a messy list of ranges.
A subnet calculator starts from a single network and netmask or prefix and works out its boundaries and host count. IP range to CIDR instead starts from an arbitrary inclusive range and finds the minimal set of CIDR blocks that covers it exactly, without adding or losing addresses.
The ASN lookup tool identifies an IP address and its announcing autonomous system number using public BGP routing data, which is generally how network operators trace an address back to an organization. It's one of the two tools on this page that query a live external data source rather than working offline.