RFC 6052 defines how an IPv4 address is embedded in the low 32 bits of an IPv6 address, letting IPv6-only networks reach IPv4 destinations through a NAT64 gateway. The well-known prefix is 64:ff9b::/96; operators may also use their own /96 prefix.
Professional-grade network calculator. IPv4 CIDR analysis with address type detection. IPv6 sequential planning and /48/56 split strategies. VLSM automatic allocation. Supernetting / CIDR aggregation. Batch lookup, subnet overlap checking, NAT64 mapping and a What's-My-IP tool. All tabular results exportable as CSV or JSON.
The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself — it can't be assigned to a device. The broadcast address is the last address in the subnet (all host bits set to 1) and is used to reach every host on that subnet at once. Everything in between is usable for hosts.
The number after the slash is how many bits, counting from the left, are fixed as the "network" portion of the address. A /24 fixes the first 24 bits, leaving 8 bits (256 addresses, 254 usable) for hosts. A smaller number after the slash means a bigger subnet; a larger number means a smaller one.
VLSM (Variable Length Subnet Masking) lets you size each subnet to what it actually needs instead of giving every department the same block. A point-to-point link needing 2 addresses shouldn't consume a /24 meant for 250 hosts — VLSM allocates the largest requirements first and packs smaller ones into the remaining space, so you waste far fewer addresses.
If you have several contiguous subnets that are all routed the same way, advertising them individually bloats routing tables. Supernetting finds the smallest single CIDR block that covers all of them, so one route can replace several — as long as the aggregate doesn't accidentally include addresses that should route elsewhere.
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