This tip is taken from Dan Stanton at Ascolta Training.
Dan is one of the countries leading instructors for Cisco Works and he's famous to boot. Recently one of my CCNA Students asked be about Dans Method and I was at a loss. Well I pinged Dan and here it is. Thanks Dan!
Bits: 1 2 3 4 5 6 7 8
Mul Octet: 128 64 32 16 8 4 2 1
Sub Mask: 128 192 224 240 248 252 254 255
Wild Mask: 127 63 31 15 7 3 1 0
The number of bits determine the multiple of the octet.
SUBNETS:
255.255.255.224 is multiples of 32 in the last octet.
Subnet Broadcast
204.10.20.0/27 204.10.20.31
204.10.20.32/27 204.10.20.63
04.10.20.64/27 204.10.20.95
etc…
255.255.248.0 is multiples of 8 in the third octet.
Subnet Broadcast
144.251.8.0/21 144.251.15.255
144.251.16.0/21 144.251.23.255
144.251.24.0/21 144.251.31.255
etc…
VLMS (A multiple of 32 can be divided into 4 multiples of
204.10.20.0/27
204.10.20.32/27 = 204.10.20.32/29
204.10.20.40/29
204.10.20.48/29
204.10.20.56/29
204.10.20.64/27
204.10.20.96/27
etc
ROUTE SUMMARIZATION (4 subnets in the summary route.)
144.251.12.0/22 = 144.251.12.0/24
144.251.13.0/24
144.251.14.0/24
144.251.15.0/24
SUPER NETTING (4 networks in the summary route.)
204.251.12.0/22 = 204.251.12.0/24
204.251.13.0/24
204.251.14.0/24
204.251.15.0/24
WILDCARD MASKS
Wild card masks are the inverse of subnet masks on contiguous subnets
255 255 255
-128 -192 -224 etc…
===== ==== ====
127 63 31
