http://www.c-ps.net/trade/content/2006/10/3416.htmlUnderstanding types of NATIt’s important to understand the different types of NAT behavior:
Reuses the same address and port when talking to all peers.
Multiple IP address, symmetric
Picks a new address and port when talking to a new peer.
Single IP address, symmetric
Uses the same address but a new port when talking to a new peer.
It’s also important to understand the filtering behaviors of NATs and firewalls:
A cone NAT with no filtering is called “full cone”.
The peer is restricted to talking only to addresses it has talked to already.
Address and port-restricted
The peer is restricted to talking only to addresses and ports it has talked to already.
In addition, some NAT and firewall behaviors aren't easily defined. For example, a NAT could act as a symmetric NAT that preserves port numbers. When it runs out of resources, it could switch and act as a cone NAT.
In another example, a NAT could act as one type of NAT for the first client that connected to a server. It could act as a different type of NAT for the second client that tried to connect to the same server. In this case, a simple analysis can fail to predict whether a client can make a peer-to-peer connection.
Note: A firewall can filter and not be a NAT. A NAT can act as a firewall and have filtering. A firewall may block UDP completely.