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===NAT is evil===
We could go on about the evils of NAT - but suffice to say that it is a bodge. It breaks the fundamental design principles of IP. It came about almost by accident as a way of handling mutliplemultiple devices when people moved from using one PC with a modem to a small network in their home. It served a vaguely useful purpose in that respect allowing some internet connection sharing which works for some things (web pages, email, etc.). However, it has a lot of problems. Many devices that do NAT also have ALGs (Application Level Gateways) which assist the NAT by tinkering with the packets as they go through in complex ways where the devices understands the higher level protocol. There are devices that do a reasonal job at this even with NAT. Even so, expecting this to work is pot luck as NAT and especially NAT ALGs are not built to any standard or even well documented. They work in some cases and not others.
===Why is SIP a problem===
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