You can do this by making a smaller subnet. So for example, if you have a /24 network, you can carve out a small block from this subnet such as a /29 network.
For example if
188.8.131.52/24 was your public network. You could use a second interface with
184.108.40.206/29. You would then have 220.127.116.11 assigned to that interface and that would be the gateway for clients. You would then have 5 remaining ips you could use.
[kbrandt@alpine: ~] ipcalc 18.104.22.168/29
Address: 22.214.171.124 00001000.00001000.00001000.11110 000
Netmask: 255.255.255.248 = 29 11111111.11111111.11111111.11111 000
Wildcard: 0.0.0.7 00000000.00000000.00000000.00000 111
Network: 126.96.36.199/29 00001000.00001000.00001000.11110 000
HostMin: 188.8.131.52 00001000.00001000.00001000.11110 001
HostMax: 184.108.40.206 00001000.00001000.00001000.11110 110
Broadcast: 220.127.116.11 00001000.00001000.00001000.11110 111
Hosts/Net: 6 Class A
Even though you might have the /24 network on a different interface that overlaps with the /29, that doesn't matter because the more specific (you might think of it as smaller) route always wins. Keep in mind that when you do this you lose 2 usable ip address, one for the network and one for the broadcast.
Learning to subnet is a worthwhile skill, we have our own sort of Mega answer on that at How does Subnetting Work?.
The other option is to not do any NAT for IP blocks and only use NAT for specific IPs. This works fine because NAT rules will take precedence as they happen before routing.