Network Tools
Tools for working with IP addressing and subnets. All processing happens in your browser, so the ranges you enter are never uploaded or looked up.
Working with network ranges
Use the subnet calculator when you need the network address, broadcast address, usable host range, mask, wildcard, or address count for an IPv4 or IPv6 prefix. It accepts CIDR notation, IPv4 netmasks, and individual host addresses.
For planning smaller networks, split a prefix into child subnets and inspect each resulting range before changing a router or firewall. The calculation is local and performs no DNS, WHOIS, or external address lookup.
One boundary drawn through 32 bits
Subnetting looks like arithmetic and is really one line drawn through an address. The prefix length says how many bits identify the network; everything to the right of that line identifies a host inside it. A netmask is the same line written as an address, and a wildcard mask is its inverse, which is the form Cisco access lists and OSPF expect.
Because the line can only sit between bits, every block is a power of two. That is why there is no subnet of 100 addresses, only 64 or 128, and why moving the boundary one place either doubles or halves the block.
Recognising a range on sight
A handful of ranges carry meaning that no calculator can tell you. 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private and unroutable; 100.64.0.0/10 is carrier-grade NAT; 169.254.0.0/16 is what a machine assigns itself when DHCP fails, so it is usually a symptom rather than a configuration.
For diagrams and documentation, the blocks reserved by RFC 5737 exist precisely so you do not have to borrow somebody's real address. Every result here labels the scope it falls into, which is often the fastest answer to why a host cannot be reached.
Plan for the renumbering you would rather avoid
Sizing each subnet to what it holds is cheap at design time and expensive afterwards. A point-to-point link needs two addresses; a user VLAN with sixty devices needs a /26; a segment that will grow needs room to grow, because renumbering means touching every device, firewall rule and static entry that referred to the old range.
Allocating on boundaries that summarise pays off in the routing table: a site that fits inside one /20 is one route to the rest of the network rather than sixteen. And in IPv6 the arithmetic is the same while the habits are not - a /64 per segment, always, with no need to conserve.