When you receive a block of addresses from an ISP, what you get will look something like
this: 192.168.10.32/28. What this is telling you is what your subnet mask is. The slash notation
(/) means how many bits are turned on (1s). Obviously, the maximum could only be /32 because
a byte is 8 bits and there are four bytes in an IP address: (4
×
8 = 32). But keep in mind that the
largest subnet mask available (regardless of the class of address) can only be a /30 because
you’ve got to keep at least 2 bits for host bits.
Take for example a Class A default subnet mask, which is 255.0.0.0. This means that the first
byte of the subnet mask is all ones (1s) or 11111111. When referring to a slash notation, you
need to count all the 1s bits to figure out your mask. The 255.0.0.0 is considered a /8 because
it has 8 bits that are 1s—that is, 8 bits that are turned on.
A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones (1s):11111111.11111111.00000000.00000000.
Subnet Mask CIDR Value
255.0.0.0 /8
255.128.0.0 /9
255.192.0.0 /10
255.224.0.0 /11
255.240.0.0 /12
255.248.0.0 /13
255.252.0.0 /14
255.254.0.0 /15
255.255.0.0 /16
255.255.128.0 /17
255.255.192.0 /18
255.255.224.0 /19
255.255.240.0 /20
255.255.248.0 /21
255.255.252.0 /22
255.255.254.0 /23
255.255.255.0 /24
255.255.255.128 /25
255.255.255.192 /26
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