What is CIDR?
CIDR (Classless Inter-Domain Routing) says how many bits of an IP are the network versus the host. 192.168.1.0/24 means the first 24 bits are the network: 256 addresses, classically 254 usable hosts after dropping network and broadcast.
This page computes IPv4 and IPv6 network address, broadcast (IPv4 only), wildcard mask, usable range, binary mask, integer/hex IDs and the PTR zone (in-addr.arpa / ip6.arpa). You need those values for VLAN plans, DHCP scopes and ACL lines.
Network, broadcast and wildcard
The network address has host bits set to zero; it names the subnet. Broadcast has host bits set to one; on IPv4 it means “everyone on this network”. Usable hosts sit between the two.
A wildcard mask is the bitwise inverse of the subnet mask (255.255.255.0 → 0.0.0.255). Cisco ACLs and OSPF network statements use wildcard, not netmask. The binary mask shows which bits are locked.
Classful addressing and VLSM
The old classful model used A /8, B /16, C /24. The first octet still names the class (1–127 A, 128–191 B, 192–223 C; multicast D, 240+ E). CIDR breaks those defaults: you can cut a “class C” into a /27 VLAN with 30 hosts (VLSM).
The A/B/C filter on the form only shortens the subnet list to prefixes at least as long as that class default. The calculation is still CIDR; class in the result is read from the first octet.
/31, /32 and special IPv4 ranges
/32 is a single host (loopback address, BGP peer). /31 (RFC 3021) gives two addresses on a point-to-point link with no network/broadcast split. /30 is the older p2p habit (two usable plus network and broadcast).
10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are RFC 1918 private and must not be announced on the internet. 100.64.0.0/10 is CGNAT (RFC 6598), 169.254.0.0/16 link-local, 127.0.0.0/8 loopback, 192.0.2.0/24 and sibling TEST-NET blocks are documentation. IPv4-mapped (::ffff:a.b.c.d) and 6to4 (2002:xxxx:xxxx::/48) are transition forms; do not use 6to4 in new designs.
IPv6 prefixes
IPv6 has no broadcast; multicast and anycast exist instead. LANs almost always use /64 for SLAAC. Operators typically give a site a /48 (or /56); each VLAN then takes a /64. Point-to-point uses /127 (RFC 6164); /128 is one address.
fc00::/7 is unique local (ULA, private IPv6), fe80::/10 link-local, 2001:db8::/32 documentation, ff00::/8 multicast, 2000::/3 global unicast. Address count is 2^(128−prefix); even a /64 holds more than 18 quintillion hosts — the “minus 2” usable rule is IPv4-only.