1. In networking, protocols are layered on top of each other, with each layer responsible for a different aspect of communication. For TCP/IP, the protocol stack looks something like: HTTP Telnet POP3 SNMP bootp TCP UDP ICMP IP ARP PPP Ethernet The way that you would use this diagram is the following paragraph: You use the protocol HTTP to request a web-page. The HTTP client (web-browser) contacts the HTTP server (web-site) using the protocol TCP. The protocol TCP segments all its work into IP packets. Routers on the Internet know how to forward the IP packets, but are clueless as to whatever is inside the IP packets. Your machine will use something like PPP or Ethernet in order to send IP packets to the nearest router. Key point: Encryption can happen at any layer. Payload The data itself can be encrypted independent of the protocols used to transport it. For example, a typical use of PGP is to encrypt a message before sending via e-mail. All the e-mail programs and protocols are totally unaware that this has occurred. Application Layer Some applications have the ability to encrypt data automatically. For example, SMB can encrypt data as it goes across the wire Transport Layer SSL is essentially encryption at the transport layer. Network Layer IPsec provides encryption at the network layer, encrypting all the contents above IP, including the TCP and UDP headers themselves. From Hacking-Lexicon