Source NAT changes the source address in IP header of a packet. It may also change the source port in the TCP/UDP headers. The typical usage is to change the a private (rfc1918) address/port into a public address/port for packets leaving your network. Available only in the POSTROUTING chain in iptables.
iptables -t nat -A POSTROUTING -i eth1 -j SNAT --to-source 188.8.131.52[:port]