AH stands for Authentication Header, and authenticates the entire packet that is transmitted - therefore, it's in-transit - tied to IPSec - The IPsec protocols AH and ESP can be implemented in a host-to-host transport mode, as well as in a network tunneling mode. AES-256 is the symmetric encryption key algorithm (block cipher) developed by NIST which is used to protect data at rest such as databases for example, or whole disk encryption.