CIDR → Rede + Broadcast

CIDR network and broadcast addresses

Take an IPv4 address with a CIDR mask. The network address falls out of a bitwise host AND mask, while the broadcast address comes from host OR (~mask). Run 192.168.1.50/24 through it and you get mask 255.255.255.0, network 192.168.1.0, broadcast 192.168.1.255, with 192.168.1.1–192.168.1.254 left over for hosts. A /26 like 10.0.5.130/26 lands on network 10.0.5.128 and broadcast 10.0.5.191. CIDR (RFC 1519) did away with the old classful A/B/C model. Prefixes can now be aggregated, which keeps the global routing table from ballooning.

Applications and context

These two addresses show up all over the place: routing decisions via longest-prefix match, firewall and ACL rules that allow or deny by subnet, VLAN configuration on switches, OSPF/BGP advertisements, sizing a DHCP scope, on-prem segmentation. The broadcast in particular matters for protocols like WoL (Wake-on-LAN) and the legacy NetBIOS discovery that still lingers in some networks.

FAQ

Can the broadcast address be assigned to a host? No. Anything sent to it floods every host on the subnet, so it can't serve as a unicast address.

What is the difference between directed and limited broadcast? A directed broadcast aims at one specific subnet, say 192.168.1.255. The limited broadcast, 255.255.255.255, never leaves the local link.

Does IPv6 have a broadcast? No. It drops the concept entirely and leans on multicast groups instead, like ff02::1 for all nodes on the link.