Certain chunks of IPv4 IP address are specially allocated by Request for Commentss for special uses such as Loopback (RFC 1643), Private networks (RFC 1918), and Zeroconf (RFC 3927) usage, and are not available for allocation by Regional Internet Registry (RIRs).

The subnetwork mask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.

Not all sizes of prefix announcement may be routable on the public Internet: see Routing, Peering.

Classless Inter-Domain RoutingNetmaskHosts / subnetclassful network nameTypical usage /8255.0.0.016777216Class ALargest block allocation made by IANA /9255.128.0.08388608 /10255.192.0.04194304 /11255.224.0.02097152 /12255.240.0.01048576 /13255.248.0.0524288 /14255.252.0.0262144 /15255.254.0.0131072 /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 /22255.255.252.01024 /23255.255.254.0512 /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 /29255.255.255.2488 /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