Hi,
I'm not sure if thats how ASAs NAT works. Atleast I havent been in that situation. But to my understanding ASA doesnt use the same NAT IP address for the same inside hosts concurrent connections. (But I cant really confirm this)
When you have these global NAT pools configured for the /16 networks, have you assigned also 1 PAT IP address to handle the situation where your NAT pool runs out? For example take some Pool range and use the last IP address of that range as PAT address.
For example the following configuration
global (outside) 192 192.168.1.100-192.168.1.199
global (outside) 192 192.168.1.200
nat (inside) 192 172.16.0.0 255.255.0.0
- Jouni