Protocol & Service
A single server can not communicate with the outside. This is why there are protocols and services.
A protocol is a serie of steps to allow communication between multiple computers or networked peripherals.
The OSI (Open Systems Interconnection) model is a standard for network communication of all computer systems. It is a model of communication between computers proposed by ISO that describes the functionalities necessary for the communication and organization of these functions.
TCP/IP (acronym for Transmission Control Protocol / Internet Protocol) is a communication protocol. TCP/IP consists of two protocols: TCP, which manages packet flows, and IP, for error checking.
Simple Network Management Protocol (SNMP) is a communication protocol that allows network administrators to manage network equipment, monitor and diagnose network and hardware problems remotely.
A service is a software element that provides reusable functionality in multiple business process contexts
Dynamic Host Configuration Protocol (DHCP) is a network protocol whose role is to automatically configure a station's IP settings, including automatically assigning it an IP address and a subnet mask, network.
The Domain Name System (DNS) is an essential system for the Internet. It is, among other things, the service that makes it possible to establish the correspondence between the domain name and its IP address.
SMTP, POP & IMAP
SMTP stands for Simple Message Transfer Protocol, which is used to forward e-mail messages over networks. An SMTP server is a service that listens on port 25, its main objective is to route mails from the address of the recipient. The Post Office Protocol (POP), which listens on port 110, has a specific purpose: to allow the user to pick up his mail from a host that does not contain his mailbox. POP establishes a dialog between the e-mail software (MUA) and the user's mailbox on the server. IMAP (Internet Message Access Protocol), which listens on port 143, is a mail retrieval protocol. IMAP4 is thus an alternative to POP3. Not only does IMAP offer more services than POP, but they are also more sophisticated. These three protocols form the e-mail services.
WEB WS-* Services refers to the software implementation of WS- * specifications and is based on a set of basic protocols and standards used for the exchange of data between applications in heterogeneous environments.
A quick summary?
What it is important to understand, that is, you are able to succeed in circumventing a security, you must know each of its cogs. Understand how it works.