Network Address Translation is not always as transparent a process as it should be. Everything would be fine if IP was the only protocol carrying IP address information. There are some protocols that send IPs as part of the data they transmit, and if this is a translated IP that is sent to a receiver behind our NAT-router we are going to have a problem. That means, the receiver will have a problem, since he can impossibly reach the host the IP transmitted has been meant to address. The only way to solve this issue is to look at the data transmitted by certain protocols known to include IP information, which of course means additional overhead and complication.