Juniper router IP assignment

I'm in a job where I configure circuits for VPN networks. I encounter both Cisco and juniper routers.

While determining the next available IP assignment, I noticed that there seemed to be a pattern of the IP assignment. They would go in increments of 4 (xx.xx.xxx.16, xx.xx.xxx.20, etc.). Within each group of four, the IP would increment by 1 (. 16 would have 17, 18, and 19 available to use and 20 would have 21, 22, and 23 to use).

I was wondering about the .16 and the .20. Are those not used?