Saying How To Mangle The Packets

So now we know how to select the packets we want to mangle. To complete our rule, we need to tell the kernel exactly what we want it to do to the packets.

6.1 Source NAT

You want to do Source NAT; change the source address of connections to something different. This is done in the POSTROUTING chain, just before it is finally sent out; this is an important detail, since it means that anything else on the Linux box itself (routing, packet filtering) will see the packet unchanged. It also means that the `-o' (outgoing interface) option can be used.
Source NAT is specified using `-j SNAT', and the `--to-source' option specifies an IP address, a range of IP addresses, and an optional port or range of ports (for UDP and TCP protocols only).

## Change source addresses to 1.2.3.4.
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4

## Change source addresses to 1.2.3.4, 1.2.3.5 or 1.2.3.6
# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4-1.2.3.6

## Change source addresses to 1.2.3.4, ports 1-1023
# iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to 1.2.3.4:1-1023

Post subject: Netmap vs dstnat & srcnat

I am about to replace an old Cisco SOHO Router with a MT RB1100 and want to make sure that I get it right. I have a /30 to the provider with a /28 routed over the /30. The Public IP's in the /28 are assigned (mapped) when needed to various private IP's in some /24 and /22 private subnets. All other IP's in the private /24 are natted to the Public /30. The Cisco didn't care which Public IP's went to which subnets or addresses as they were done with one rule for each mapping as below.