Subnet / CIDR Notation
Definition
A subnet is a logical subdivision of an IP network, written in CIDR (Classless Inter-Domain Routing) notation as an address followed by the prefix length. In IPv4, a /24 provides 254 usable host addresses, a /23 offers 510, a /22 1022 and a /21 2046, because the network address and the broadcast address are reserved. A /16 provides 65,534. All hosts of a subnet share one broadcast domain, so ARP and other broadcast traffic grows with the number of devices. Two devices on different subnets communicate only through a router. Private networks use the RFC 1918 ranges 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, and devices that find no DHCP server, Dante devices among them, fall back to link-local addresses in 169.254.0.0/16 (RFC 3927).
In the field
A common addressing error places one device in 192.168.1.x/24 and another in 192.168.2.x/24: the two are on different subnets and cannot reach each other directly. A mismatched subnet mask produces less obvious faults, because two devices then disagree about whether they share a subnet. Production AV networks are usually planned with one subnet per service or VLAN, often a /24, sized to the number of devices.
Related lessons
Sources
- RFC 4632 : notation CIDR et longueur de préfixe · rfc-editor.org
- RFC 1918 §3 : blocs privés 10/8, 172.16/12, 192.168/16 · rfc-editor.org
- RFC 3927 : adresses IPv4 link-local en 169.254/16 · rfc-editor.org
- Audinate, Dante Controller User Guide, Troubleshooting Dante IP Address Configuration : DHCP par défaut, repli link-local · dev.audinate.com
- RFC 826 : ARP, requêtes diffusées en broadcast sur le segment · rfc-editor.org
- RFC 1122 §3.2.1.3 : numéro d'hôte tout à 0 ou tout à 1 réservé (adresse du réseau, broadcast), jamais attribué à un hôte · rfc-editor.org