NAT can take a single IP address and distribute it for use of many users by creating a Local Area Network (LAN).The LAN will share a single globally recognized IP address.
Disadvantages:

Translation sometime complex (e.g. FTP, VOIP).
Apps using dynamic ports (UPnP).
Does not scale (today avg. of 500 active sessions per user).
Introduce states inside the network:
Multi-homed networks
Breaks [...]