The subnet is made by the fist 3 numbers of an IP.

For instance, if you have this IP,  that belongs to the subnet 11.22.33. Also all the IPs from to are in the same subnet 11.22.33.XXX .

Having proxies from more subnets, is usually a good thing because you will have IP diversity and it will mean less bans because some sites will ban entire subnets.