It defines the IP range of the devices on the VPN to be a subset of the special/reserved "private network" range [0]. More specifically it says that the VPN IP

It defines the IP range of the devices on the VPN to be a subset of the special/reserved "private network" range [0]. More specifically it says that the VPN IP's are over the range of 10.10.10.0 to 10.10.10.255. This can be calculated from taking the binary representation of the specified IP and 'fixing' the first 24 (in this case) bits, the range of possible addresses is the range over which the last 8 (in this case) bits can vary. It is common to also see a broadcast address, gateway, and network mask specified. [0] https://en.wikipedia.org/wiki/IPv4#Special-use_addresses https://en.wikipedia.org/wiki/IPv4#Special-use_addresses