acl summarisation

oxzgan

i got a question on acl summarisation
first example
summarise these networks
192.168.32.0/24
192.168.33.0/24
192.168.34.0/24
192.168.35.0/24
192.168.36.0/24
192.168.37.0/24
192.168.38.0/24
192.168.39.0/24

i know the answer for this this can summarised to
192.168.32.0 and wild card mask 0.0.7.255
ie 192.168.32.0/21 ie subnet mask 255.255.248.0
( i used the trick 39-32 =7 , so 0.0.7.255 wild card mask )


here is is my next example that arises my confusion :
192.168.146.0/24
192.168.147.0/24
192.168.148.0/24
192.168.149.0/24

why i can't summarise it to 192.168.144.0/21 or 192.168.146.0/21?
why i can't use the trick 149-146 =3 and wild card mask 0.0.3.255 here?

the book says answer is 192.168.146.0 and wild card 0.0.1.255 and
192.168.148.0 and wild card 0.0.1.255

oxzgan

i think i found answer ...if we use 192.168.144.0/21
this will include ip ranges from. 192.168.144.0 to 192.168.151.0 , this means it includes lot if un wanted ranges like 192.168.144.0, 192.168.145.0 , 192.168.150.0 , 192.168.151.0 . So this means wastage of ips.
May be this is is why the book i study don't use answer 192.168.144.0/21

am i correct?

also got one question in above example ( first example summarisation of 192.168.32.0/24 we simply solved by substracting trick .why i can't use this trick here and i had to use traditional way of summarisation ( checking similar bits )