A packet is the unit of transmission on a physical network.


A datagram is the unit of transmission in the IP protocol. To cross a particular network a datagram is encapsulated inside a packet.


A router is a switch that receives data transmission units from input interfaces and, depending on the addresses in those units, routes them to the appropriate output interfaces. There can be routers at different levels of protocol. For example, Interface Message Processors (IMPs) are packet-level routers.


In the Internet documentation generally, and in this document specifically, a gateway is an IP-level router. In the Internet community the term has a long history of this usage.

Internet Gateways

An Internet gateway is an IP-level router that performs the following functions:
  1. Conforms to specific Internet protocols specified in this document, including the Internet Protocol (IP), Internet Control Message Protocol (ICMP), and others as necessary.
  2. Interfaces to two or more packet networks. For each connected network the gateway must implement the functions required by that network. These functions typically include:
  3. Receives and forwards Internet datagrams. Important issues are buffer management, congestion control, and fairness.
  4. Chooses a next-hop destination for each IP datagram, based on the information in its routing data-base.
  5. Supports an interior gateway protocol (IGP) to carry out distributed routing and reachability algorithms with the other gateways in the same autonomous system. In addition, some gateways will need to support the Exterior Gateway Protocol (EGP) to exchange topological information with other autonomous systems.
  6. Provides system support facilities, including loading, debugging, status reporting, exception reporting and control.


A domain is a collection of hosts and routers that use the same routing protocol and are administered by a single authority. In other words,a domain might be an internetwork administered by a university or other organization. 


The internet is divided into domains, or autonomous systems. Interior Gateway Protocols(IGPs) are the protocols used within a domain for the exchange of routing information. 


The Exterior Gateway Protocol(EGP) provides a way for two neighboring routers located at edges of their respective domains to exchange messages and information. The Extrior Gateway Protocol provides a way for routers to exchange routing information among themselves.Each domain has one or more routers that are picked to be EGPs.
EGP is rarely used these days. BGP is the exterior gateway protocol in use. 


Border Gateway Protocol(BGP) was implemented as an interim solution to provide some limited policy features, but it does not solve the scalability requirements.
BGP is the exterior gateway protocol in use Route attributes such as the cost or security of a path are also added. BGP reduces the bandwidth required to exchange routing information because the information is exchanged incrementally, rather than by sending the entire database.


A host, in the Internet is any networked device. 


Internet Control Message Protocol(ICMP) is the error and control message protocol used by the Internet protocol family. It is used by the kernel to handle and report errors in protocol processing.
ICMP is an unreliable datagram protocol layered above IP. It is used internally by the protcol code for various purposes including routing, fault isolation, and congestion control. Receipt of an ICMP "redirect" message will add a new entry in the routing table, or modify an existing one. ICMP messages are routinely sent by the protocol code. Received ICMP messages may be reflected back to users of higher-level protocols such as TCP or UDP as error returns from system calls. A copy of all ICMP message received by the system is provided using the ICMP raw socket. 


The User Datagram Protocol(UDP) is part of the TCP/IP protocol suite. It was created to provide a way for applications to access the connectionless features of IP. Both TCP and UDP use IP. UDP was designed to allow appplications to create daragrams and address them to the ports for accessing applications or processes. UDP's primary role is to add the port address of an application process to an IP packet. 


Authentication assigns a unique identification to each user for each logon session. The identification, not the user's password, is used to authenticate each of the user's network requests. Authentication guarantees that a user's password never goes beyond the logon process. It is immediately conveted to a different code that identifies the user and the station they are logged into during the user's currrent session. Authentication also guarantees that messages are from the correct user at his or her workstation in the current session and not corrupted, counterfeited, or tampered with. 


Sockets were originally local interprocess communication mechanisms used in the UNIX environment. They evolved into network links on Transmission Control Protocol/Internet Protocol(TCP/IP) networks. A socket is basically an end point of a communication link between two applications. Sockets that extend over a network connect two or more applications running on separate computers attached to the network. A socket is composed of two addresses: Port address- This is the address for the specific application or process running within a computer. Internet Protocol(IP) address- This is the address of the workstation on the TCP/IP network. Sockets provide a full-duplex communication channel between one or more systems. A local port makes a connection to a remote socket. The significance of this is that the socket identifies a computer on the network and the software port withen that computer for a process being run by an application. Once a channel is opened, information is sent or received, and the circuit is then dismantled.


Address Resolution Protocol(ARP) is the neighbor discovery protocol for Internet and TCP/IP networks. It is similar to the OSI End Systim-to- Intermediate System(ES-IS) protocol. Both routers and host (user computers, servers, and so on) use ARP to announce themselves. A router broadcasts packets that contain an IP address. The computer or device attached to a network with the address then returns its LAN address. The information is then placed in routing tables for future use. A similar protocol, called Reverse ARP(RARP), performs the opposite task;it obtains the IP address from a known network address.


Short for Simple Network Management Protocol, a set of protocols for managing complex networks. The first versions of SNMP were developed in the early 80s. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters.
SNMP 1 reports only whether a device is functioning properly. The industry has attempted to define a new set of protocols called SNMP 2 that would provide additional information, but the standardization efforts have not been successful. Instead, network managers have turned to a related technology called RMON that provides more detailed information about network usage.


A computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN).
Most LANs connect workstations and personal computers. Each node (individual computer ) in a LAN has its own CPU with which it executes programs, but it is also able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.

There are many different types of LANs Ethernets being the most common for PCs. Most Apple Macintosh networks are based on Apple's AppleTalk network system, which is built into Macintosh computers.


A computer network that spans a relatively large geographical area. Typically, a WAN consists of two or more local-area networks (LANs).
Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet.