In computer network protocol design, inter-server communication is an extension of the client–server model in which data are exchanged directly between server

Server (computing)

In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

s. In some fields server-to-server (S2S) is used as an alternative, and the term inter-domain

Inter-domain

inter-domain is a term used to describe data flow control and interaction between Primary Domain Controller computers. This type of computer uses various computer protocols and services to operate. It is most commonly used to multicast between internet domains.-Internet use:An Internet service...

can in some cases be used interchangeably.

Protocols that have inter-server functions as well as the regular client–server communications include the following:

• The domain name system
  Domain name system
  The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
  
   (DNS), which uses an inter-server protocol for zone transfers
  DNS zone transfer
  DNS zone transfer, also sometimes known by its opcode mnemonic AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to employ for replicating the databases containing the DNS data across a set of DNS servers. Zone transfer comes in two flavors, full ...
  
  ;
• The Dynamic Host Configuration Protocol
  Dynamic Host Configuration Protocol
  The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
  
   (DHCP);
• FXP
  File eXchange Protocol
  File eXchange Protocol and is a method of data transfer which uses FTP to transfer data from one remote server to another without routing this data through the client's connection. Conventional FTP involves a single server and a single client; all data transmission is done between these two...
  
  , allowing file transfer directly between FTP
  File Transfer Protocol
  File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
  
   servers;
• The Inter-Asterisk eXchange (IAX);
• InterMUD
  InterMUD
  InterMUD or interMU* communications is the commonly accepted terminology for different methods of allowing MUDs to communicate with each other. Some of the more common of these methods are custom protocols over the internet, IPC messages, and bots. The custom protocol method, which is by far the...
  
  ;
• The IRC
  Internet Relay Chat
  Internet Relay Chat is a protocol for real-time Internet text messaging or synchronous conferencing. It is mainly designed for group communication in discussion forums, called channels, but also allows one-to-one communication via private message as well as chat and data transfer, including file...
  
  , an Internet chat system with an inter-server protocol allowing clients to be distributed across many servers;
• The Network News Transfer Protocol
  Network News Transfer Protocol
  The Network News Transfer Protocol is an Internet application protocol used for transporting Usenet news articles between news servers and for reading and posting articles by end user client applications...
  
   (NNTP);
• The Protocol for SYnchronous Conferencing (PSYC);
• SIP
  Session Initiation Protocol
  The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
  
  , a signaling protocol commonly used for Voice over IP
  Voice over IP
  Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
  
  ;
• SILC
  SILC (protocol)
  SILC is a protocol that provides secure synchronous conferencing services over the Internet.- Components :...
  
  , a secure Internet
  Internet
  The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
  
   conferencing
  Synchronous conferencing
  Synchronous conferencing is the formal term used in science, in particular in computer-mediated communication, collaboration and learning, to describe online chat technologies. It has arisen at a time when the term chat had a negative connotation...
  
   protocol;
• The Extensible Messaging and Presence Protocol
  Extensible Messaging and Presence Protocol
  Extensible Messaging and Presence Protocol is an open-standard communications protocol for message-oriented middleware based on XML . The protocol was originally named Jabber, and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging , presence...
  
   (XMPP, formerly named Jabber).

Some of these protocols employ multicast

Multicast

In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...

strategies to efficiently deliver information to multiple servers at once.