IPv4 CIDR Subnet Calculator & Bitwise Inspector
Classless Inter-Domain Routing • Subnet Masks • Host Ranges & Wildcards
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Frequently Asked Questions & Security Specifications
What is CIDR and why is it used over legacy IP classes?
CIDR (Classless Inter-Domain Routing, RFC 1519) replaced restrictive Class A, B, and C networks. It allows variable-length subnet masking (VLSM) to allocate IP address blocks with granular efficiency, preventing address exhaustion.
Why are the first and last addresses in a subnet unusable for hosts?
In any standard IPv4 network, the first address (all host bits 0) represents the Network ID, and the last address (all host bits 1) is reserved as the Subnet Broadcast Address. Thus, usable hosts = 2^(32 - prefix) - 2.
What is a wildcard mask?
A wildcard mask (used in Cisco ACLs and OSPF) is the bitwise inverse of the subnet mask (`255.255.255.255 - SubnetMask`). 0 indicates an exact bit match, while 1 indicates wildcard bits.