You have the Netscreen on the network. It doesn't matter about subnets since you have all on the same network and no DHCP server.
That can create problems as the default gateway must be laid out as well.
-> I do see a problem in your basic IP assignments as you are not using private addressing inside your network?
First a little background.
I have two subnets at my office connected to the internet:
220.127.116.11/30 (Mask 255.255.255.252) - 2 Hosts
18.104.22.168/27 (Mask 255.255.255.224) - 30 Hosts
22.214.171.124 is assigned to the external interface of my firewall. 126.96.36.199 is my firewall's default gateway (My ISP's router). I do not have a router I control in front of my firewall.
The 188.8.131.52 subnet is routed to my network by my isp. I have 184.108.40.206 assigned to a NIC on my firewall so I can take the other addresses and use static NAT to servers I am providing services to (i.e. mail, ftp, etc). If I tracert from any device with a static nat it's first hop outside my network is on 220.127.116.11.
Here is where the question comes in. I have a client that is bringing their own firewall over to put on the network since my existing one I cannot get VPN to work properly. My ISP doesn't have any IPs from the /27 setup on their routers as far as I know. My initial thought was to take an IP from my routed net to put on their firewall, for example 18.104.22.168. The only device with a physical IP on this net is my firewall which I cannot use as the default gateway. So what would I configure on the client firewall for the gateway?
My thought was to call my ISP and see if the can assign 22.214.171.124 to their internet connected router so I can use that as the default gateway. Is this the way to get this to work?