Cómo expandir un rango de IPs o CIDR

$ sipcalc 10.0.10.4/20
-[ipv4 : 10.0.10.4/20] - 0

[CIDR]
Host address - 10.0.10.4
Host address (decimal) - 167774724
Host address (hex) - A000A04
Network address - 10.0.0.0
Network mask - 255.255.240.0
Network mask (bits) - 20
Network mask (hex) - FFFFF000
Broadcast address - 10.0.15.255
Cisco wildcard - 0.0.15.255
Addresses in network - 4096
Network range - 10.0.0.0 - 10.0.15.255
Usable range - 10.0.0.1 - 10.0.15.254

O, si prefieres algo más bonito, puedes usar esta calculadora online.