Using multiple public IPs on WAN interface

Some users get from their ISP more than one routable public IP address.
In this case you need to assign these IPs to new WAN interfaces and then add a DNAT and SNAT rule in firewall.
In our example we assume that our ISP assigned us 100.64.0.0/29 (or 255.255.255.248) with gateway 100.64.0.1. WAN interface already has the first available IP, 100.64.0.2, so we'll add .3, till .6 and they will be assigned to internal hosts on IPs 192.168.1.x.

Add a new WAN interface:

# /etc/config/network
 
config interface 'wan3'
        option proto 'static'
        option device 'eth1'
        option ipaddr '100.64.0.3'
        option netmask '255.255.255.248'
        option gateway '100.64.0.1'

Add DNAT (for incoming) and SNAT (for outgoing) connections:

# /etc/config/firewall
 
config redirect
        option name 'dnat3'
        option src 'wan'
        option src_dip '100.64.0.3'
        option dest 'lan'
        option dest_ip '192.168.1.3'
        option proto 'all'
        option target 'DNAT'
 
config redirect
        option name 'snat3'
        option src 'lan'
        option src_ip '192.168.1.3'
        option src_dip '100.64.0.3'
        option dest 'wan'
        option proto 'all'
        option target 'SNAT'

Restart network and firewall services.

/etc/init.d/network restart
/etc/init.d/firewall restart