how to route my public IP

[runlevel 3]

hello, i'm new to this freebsd, my office has 16 public IP and divide to 4
branch, each branch has 3 public ip's.
my problem is, I want to make all branch connect to the main office's
router, but what I found is, all branch is showed by the internet using one
public IP (my main router IP) not their own Public IP. Anyone can help me
resolve this ?

Regards,

Dzak