Chapter 16 SSL VPN for FortiOS 5.0 : Basic Configuration : User accounts and groups : IP addresses for users
  
IP addresses for users
After the FortiGate unit authenticates a request for a tunnel-mode connection, the FortiGate unit assigns the SSL VPN client an IP address for the session. The address is assigned from an address range (IP Pool) which is a firewall address that defines an IP address range.
 
Take care to prevent overlapping IP addresses. Do not assign to clients any IP addresses that are already in use on the private network. As a precaution, consider assigning IP addresses from a network that is not commonly used (for example, 10.254.254.0/24).
To set tunnel-mode client IP address range - web-based manager
1. Go to Firewall Objects > Address > Addresses and select Create New.
2. Enter an Name, for example, SSL_VPN_tunnel_range.
3. Select a Type of IP Range.
4. In the Subnet/IP Range field, enter the starting and ending IP addresses that you want to assign to SSL VPN clients, for example 10.254.254.[80-100].
5. In Interface, select Any.
6. Select OK.
To set tunnel-mode client IP address range - CLI
If your SSL VPN tunnel range is for example 10.254.254.80 - 10.254.254.100, you could enter
config firewall address
edit SSL_tunnel_users
set type iprange
set end-ip 10.254.254.100
set start-ip 10.254.254.80
end
end
You can select the tunnel-mode IP Pools in two places:
The VPN > SSL > Config page IP Pools setting applies to all web portals that do not specify their own IP Pools.
The web portal Tunnel Mode widget IP Pools setting, if used, applies only to the web portal and overrides the setting in VPN > SSL > Config. See “Tunnel mode and split tunneling”.