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IP Cheat Chart

CIDR stands for, Classless Inter Domain Routing. CIDR is a way to allocate and specify the Internet addresses used in inter-domain router more flexibly than with the original system of Internet Protocol address classes. As a result, the number of available Internet addresses has been greatly increased. CIDR is now the routing system used by virtually all gateway hosts on the Internet's backbone network. Many small companies also benefit from CIDR in that 4 Class C's can be grouped into one subnet to provide over 1000 IP Addresses.

Here is a quick reference IP numbering chart.

Network TypeSubnet MaskNumber of IP’s
/ 00.0.0.04 billion
/ 1128.0.0.02 billion
/ 2192.0.0.01billion
/ 3224.0.0.0500 million
/ 4240.0.0.0250 million
/ 5248.0.0.0128 million
/ 6252.0.0.064 million
/ 7254.0.0.032 million
/ 8255.0.0.016 million
/ 9255.128.0.08 million
/ 10255.192.0.04 million
/ 11255.224.0.02 million
/ 12255.240.0.01 million
/ 13255.248.0.0524288
/ 14255.252.0.0262144
/ 15255.254.0.0131072
/ 16255.255.0.065536
/ 17255.255.128.032768
/ 18255.255.192.016384
/ 19255.255.224.08192
/ 20255.255.240.04096
/ 21255.255.248.02048
/ 22255.255.252.01024
/ 23255.255.254.0512
/ 24255.255.255.0256
/ 25255.255.255.128128
/ 26255.255.255.19264
/ 27255.255.255.22432
/ 28255.255.255.24016
/ 29255.255.255.2488
/ 30255.255.255.2524
/ 31255.255.255.2542
/ 32255.255.255.2551

The first few on the list would be very difficult to manage network and the last two meaningless subnet mask which would serve no purpose since the only two ip addresses available are taken by the network number and broadcast address.