Subnet Mask Reference Table


Subnet size
(# of hosts)
Subnet Mask First host in block
2 hosts 255.255.255.252 1,5,9,13,17,21,25,….
3 – 6 255.255.255.248 1,9,17,25,33,…
7 – 15 255.255.255.240 1,17,33,49,65,…
16 – 31 255.255.255.224 1,33,65,97,129,…
32 – 63 255.255.255.192 1,65,129,193,…
64 – 127 255.255.255.128 1 & 129
128 – 255 255.255.255.0 1
256 – 511 255.255.254.0 1,5,9,13,17,21,25,….
512 – 1023 255.255.252.0 1,9,17,25,33,…

CLUE: Referring to the table above, you can see that for sizes above 256 the 3rd segment of the subnet mask begins to countdown in the exact same pattern as how the 4th segment counted down for subnets of sizes 2 – 255.


Another table
subnettingcfe8


And another provided by CISCO
3an

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