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Ipv4 subnetting reference

CIDRNetmaskHosts / subnetClassfull nameTypical Usage
/8255.0.0.016777216Class ALargest block allocation made by IANA
/16255.255.0.065536Class B
/17255.255.128.032768ISP / large business
/18255.255.192.016384ISP / large business
/19255.255.224.08192ISP / large business
/20255.255.240.04096Small ISP / large business
/21255.255.248.02048Small ISP / large business
/24255.255.255.0256Class C Large LAN
/25255.255.255.128128 Large LAN
/26255.255.255.19264 Small LAN
/27255.255.255.22432 Small LAN
/28255.255.255.24016 Small LAN
/30255.255.255.2524"Glue network" (point to point links)
/31255.255.255.2542"Useless Network", proposed for point to point links (RFC 3021)
/32255.255.255.2551Host route

Note that certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback, RFC 1918 networks, and Zeroconf usage, and are not available for allocation by RIRs.

Note also that not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.