Why would want to do this?
Doesn't make sense to me to want to have 172.18.0.0/16 which gives you 172.18.0.0 to 172.18.255.255 and then subtract 172.18.254.0/29 which gives you 172.18.254.0 to .7
Its like you want to invalidate a range of ips. How do you expect to use 172.18.254.8 through 172.18.255.255? Use /29 for each of the remaining subnets?
I would also point out it is unusual to subnet this way. Usual way would be to take 172.18.0.0 and carve it up with say /17 which gives you a range of 172.18.0.0 - 172.18.127.255
This leaves you with another /17 range of 172.18.128.0 - 172.18.255.255
You can do half and two quarters or half and 4 eights, etc...
The idea is to be able to divide up the entire range into usable parts. This isn't what you are doing.
Edited by Wand3r3r, 09 July 2016 - 10:35 AM.