What the slash means
The number after the slash is how many leading bits identify the network. /24 fixes the first 24 bits, leaving 8 for hosts — 256 addresses, of which 254 are usable. Every step down doubles the block: /23 is 512 addresses, /22 is 1024.
Two addresses in each block are reserved: the first is the network itself and the last is broadcast. That is why a /30 point-to-point link has four addresses and only two usable ones, and why /31 exists as a special case for exactly that situation.
Where subnets collide
Overlapping ranges are the usual cause of a VPN that connects and routes nothing. If the office uses 192.168.1.0/24 and so does the home router, traffic for the far side never leaves the local network.
Choosing something unusual from 10.0.0.0/8 for anything that will ever be reached over a VPN avoids a class of problem that is painful to diagnose after the fact.
Frequently asked questions
Why does a /24 have 254 usable addresses and not 256?
The first address is the network itself and the last is broadcast. Neither can be assigned to a host.
What size subnet do I need?
Count the hosts, add room to grow, then take the next power of two above it and subtract two. Sixty devices needs a /26, which gives 62 usable.
Why does my VPN connect but not route?
Usually overlapping subnets — both ends using 192.168.1.0/24. Traffic for the far side is treated as local and never leaves.
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