I will preface this with that I always say: do not implement this without IPv6 unless you literally have no other choice. IPv6 will allow for a significant resource offload because most eyeball services (Netflix, Youtube, Google, Facebook, etc.) will prefer IPv6, thus removing your requirement for more IPv4 NAT state and overload / port utilization.
Because I found no simple how-to for using NETMAP for CGN on a Mikrotik, here is one. This simple configuration will map all TCP ports 1024-9087 to 198.51.100.0/24 and all UDP ports from 1024-9087 to 192.0.2.0/24. This is a very simple configuration that can be adjusted and further refined with address-lists for the to-addresses, etc.
This was tested on an RB2004 running ROS 7.8
/ip firewall address-list add address=100.64.0.0/21 comment=“CGN Client block 1” list=cgn1
/ip firewall nat add action=netmap chain=srcnat comment=“TCP: NetMap TCP CGNAT Rule” out-interface=WAN protocol=tcp src-address-list=cgn1 to-addresses=198.51.100.0/24 to-ports=1024-9087
/ip firewall nat add action=netmap chain=srcnat comment=“UDP: NetMap UDP CGNAT Rule” out-interface=WAN protocol=udp src-address-list=cgn1 to-addresses=192.0.2.0/24 to-ports=1024-9087
This will essentially create a 4:1 ratio of client to NAT considering UDP and TCP as equal, which they will not be.
/ip firewall nat add action=masquerade chain=srcnat comment=“Masquerade to WAN Rule” out-interface=WAN src-address-list=masquerade-list
It is also possible to offload NAT translations into hardware if your platform supports it, and requires some changes to the internal switch chip.
Assuming switch 0:
/interface/ethernet/switch set 0 l3-hw-offloading=yes/interface/ethernet/switch/port set [find] l3-hw-offloading=yes/interface/ethernet/switch/port set sfp-sfpplus2 l3-hw-offloading=no
And add the firewall filter rule:/ip firewall filter add action=fasttrack-connection chain=forward comment=“Hardware Offload for IPv4 Fasttrack” connection-state=established,related hw-offload=yes
There may be better ways to do this, but this worked in the case I needed it for.