What maps multiple internal IP addresses to a shared external IP address?

Network Address Translation, or NAT, serves the purpose of mapping multiple internal IP addresses to a single external IP address. This technique is crucial for conserving the limited number of IP addresses available in the IPv4 addressing scheme, by allowing multiple devices on a local network to communicate with external networks using a single public IP address.

When devices on an internal network send traffic to the Internet, NAT translates their private IP addresses into the shared public IP address, thereby facilitating communication while keeping internal addressing hidden. This offers an added layer of security since external entities cannot directly see the internal IP addresses.

Additionally, NAT can assist in managing portions of the routing process by tracking active sessions and ensuring that responses from external servers return to the correct internal device. This functionality is vital in both home networks and enterprise setups, where secure and efficient Internet access is essential.

The other options do not perform this specific function. For instance, DNS is used for resolving domain names to IP addresses, while proxies serve as intermediaries in client-server communication, and firewalls are primarily concerned with filtering network traffic to maintain security rather than managing IP address translation.