Subnet maths with the Cisco config already written. Plan a block with VLSM, find the tightest summary route and how much extra space it swallows, and decode any wildcard mask, including the odd non-contiguous ones you find in old ACLs.
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Config lines
Split it
Summary config
Decode a wildcard
Try
One ACL line for a list of hosts
Three things that catch people out
Summaries swallow space
10.40.0.0/22 looks tidy, but if 10.40.3.0 does not exist the summary still attracts its traffic. Add the Null0 route so it is dropped at the summarising router, not looped.
IOS rewrites your ACL
Type permit 10.1.1.9 0.0.0.255 and the device saves 10.1.1.0 0.0.0.255. Harmless here, confusing in a diff, and a sign the line was written by hand.
172.16/12 stops at 172.31
172.32.0.0 and above are public addresses. So is 100.64.0.0/10, which is reserved for carrier NAT and should not be used inside an office LAN.