IPv4 CIDR Subnet Mask Notation

The CIDR number comes from the number of 1’s in the subnet mask when converted to binary.

The common subnet mask 255.255.255.0 is 11111111.11111111.11111111.00000000 in binary.

This adds up to 24 1’s, or /24 (pronounced ‘slash twenty four’).

A subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 in binary, or 26 1’s, hence a /26.

so how to calculate the range and amounts of addresses available in that range:

valid values:

for subnet masks only those values are valid:

00000000 Bin = 0 Dec  
11000000 Bin = 192 Dec
11100000 Bin = 224 Dec
11110000 Bin = 248 Dec
11111000 Bin = 252 Dec
11111100 Bin = 254 Dec
11111111 Bin = 255 Dec

Video:

Calculator:

http://jodies.de/ipcalc

Address:   192.168.0.1           1100ffff.10101000.00ffff00 .00ffff01
Netmask:   255.255.255.0 = 24    11111111.11111111.11111111 .00ffff00
Wildcard:  0.0.0.255             00ffff00.00ffff00.00ffff00 .11111111
=>
Network:   192.168.0.0/24        11000000.10101000.00ffff00 .00ffff00 (Class C)
Broadcast: 192.168.0.255         1100ffff.10101000.00ffff00 .11111111
HostMin:   192.168.0.1           1100ffff.10101000.00ffff00 .00ffff01
HostMax:   192.168.0.254         1100ffff.10101000.00ffff00 .11111110
Hosts/Net: 254                   (Private Internet)

another example subnetmask: 255.255.255.240 = 28
allows a range of 14 unique IPv4 addresses in the subnet.



Address:   192.168.2.128         1100ffff.10101000.00ffff10.1000 0000
Netmask:   255.255.255.240 = 28  11111111.11111111.11111111.1111 0000
Wildcard:  0.0.0.15              00ffff00.00ffff00.00ffff00.0000 1111
=>
Network:   192.168.2.128/28      11000000.10101000.00ffff10.1000 0000 (Class C)
Broadcast: 192.168.2.143         1100ffff.10101000.00ffff10.1000 1111
HostMin:   192.168.2.129         1100ffff.10101000.00ffff10.1000 0001
HostMax:   192.168.2.142         1100ffff.10101000.00ffff10.1000 1110
Hosts/Net: 14                    (Private Internet)

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