Final answer:
PAT (Port Address Translation) is the most suitable NAT method for a scenario with 1000 hosts but only 20 simultaneous Internet sessions since it efficiently conserves IP addresses and provides security through address hiding.
Step-by-step explanation:
NAT Translation Method for Medium-sized Company
If you are the network administrator for a medium-sized company that owns a Class C IP address block but do not have enough registered IP addresses for the 1000 hosts in your network, and considering that no more than 20 hosts will be accessing the Internet simultaneously, the most suitable form of Network Address Translation (NAT) would be PAT (Port Address Translation).
PAT allows multiple devices on a local network to be mapped to a single public IP address but with a different port number for each session. This method is highly efficient for situations where a large number of hosts on a private network access the Internet but do not all do so at the same time. Since you have more than a hundred hosts but will only have up to 20 concurrent internet sessions, this conserves IP addresses effectively. Other options like Static NAT or Dynamic NAT wouldn't be as efficient or effective given the constraints you have mentioned.
With PAT, since only a single or a few public IP addresses are required, this translates into a considerable saving of IP addresses, which is crucial when managing a limited block of IPs as in the case with a Class C address range. Moreover, PAT also adds a level of security through its inherent characteristic of address hiding, making it difficult for outside entities to map internal network structures.