The /<number> indicates the "bitmask" of the subnet. It is the number of
1s that are in the initial part of a subnet mask.
IP space is divided into four "octets" of eight bits each (thus the name
octet). A single host netmask is 255.255.255.255, or 11111111 11111111
11111111 11111111 (32 bits). Thus, you get 123.123.123.123/32. A
"class-c" subnet is 255.255.255.0, or 11111111 11111111 11111111 00000000
(24 bits). Thus you get 123.123.123.123/24. Each time you *increase* the
bitmask by one, the address space you're referring to *halves*. 25 bits is
half of the space of a 24 (128 addresses), 26 is half of a 25 (64
addresses), and so on.
The advantage of this numbering scheme is you can represent a large amount
of information in a relatively small screen space. It's cumbersome to
write 123.123.123.123 255.255.255.0 when you can simply write
123.123.123.123/24 and mean the same thing.
Chad Scott
Network Operations
Internet Direct, Incorporated
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