The document introduces Network Address Translation (NAT), explaining its function in routing non-routable IP addresses by transforming private addresses into public ones for network communication. It details two main categories of NAT: Static NAT (SNAT), which assigns a specific public IP for each private IP, and Dynamic NAT (DNAT), which assigns public IPs from a reusable pool. Additionally, it describes Port Address Translation (PAT) as a method to enhance DNAT's scalability by allocating port numbers along with public IPs.