Dividing a given network in 4 subnets

Question

Lets suppose I get given the network (57.70.32.0/21) and I need to divide it into 4 equally sized networks.

I know the mask at the moment is (255.255.248.0) but would I need to change the prefix length of the network given to 26 so I can get the 4 equal size subnets? Basically that's my biggest question and also if I should then use the 4th octet for subnetting or stay with the 3rd to do it.

Any help would be extremely appreciated.

Answer 1

you can divide your ips into 4 networks with 510 IPs each, not counting gateway, broadcast, and net id.

each net will have a cidr of /23 which would equate to subnet 255.255.254.0

you could divide them into subnets as follows:

57.70.32.0/23 
     first address: 57.70.32.0
     last address:  57.70.33.255

57.70.34.0/23
    first address: 57.70.34.0
    last address:  57.70.35.255

57.70.36.0/23
    first address: 57.70.36.0
    last address:  57.70.37.255

57.70.38.0/23
    first address: 57.70.38.0
    last address:  57.70.39.255