/22 how to subnet, find masks etc.

[kam2700]

Thanks guys I think I've got my head around this now.

The key to understanding and working out netmasks is knowing / focusing on you can only ever have 32 total bits.


"Yes, /24 = 24 bits turned on. Of the 32 total bits, 8 bits are turned off (zeros). Your mask would be 255.255.255.0"


I can also see why Todd is saying if you learn the "what do you know" you can apply that knowledge across all the classes. This is because the masks are always the same:


Netmasks are always one of these 128,192,224,240,248,252


Depending on the prefix ie /24 /30 one of the above masks will be used. Might be in the 3rd octet for class B or the 4th octet for class C addresses.

This table from Todd's book pp 117 can show this :




TABLE 3 . 2
CIDR Values
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
255.255.255.224 /27
255.255.255.240 /28
255.255.255.248 /29
255.255.255.252 /30



Sincere thanks to Ltat42a and Fuzz.

Dan