Useful Definitions 1-3
Introduction
Active open
A sequence of events occurring when an entity using an application protocol of
the Internet suite (such as SMTP—the E-mail protocol; FTP—File Transfer
Protocol; or Telnet—terminal service protocol) directs the Transmission Control
Protocol to establish a connection over the physical medium with another user(s)
of the application’s particular service. See Transmission Control Protocol (TCP),
page 1-6, for more information.
Address mask
A bit mask that is used to select bits from an IP address for subnet addressing. The
mask is 32 bits long and selects the network portion of the IP address and one or
more bits of the local portion. See Subnet mask, page1-6, for more information.
Address Resolution Protocol (ARP)
The Internet protocol that dynamically maps destination IP addresses to physical
media (Ethernet and other) addresses. This is needed so that a datagram
addressed to logical address can reach the correct physical media address.
If the addresses are already mapped in transmitting device’s ARP cache (address
matching tables), the datagram can be sent directly.
Broadcast address
A physical or IP address referring to all stations on the media.
Connection
A logical binding between two or more users of a service so that data can be
transferred.
Connection-less mode
A service that has a single phase which combines both transmission control
mechanisms (e.g., addressing) and data transfer.
Connection-oriented mode
A service that divides into three phases: establishment, in which two or more
users are bound to a connection; data transfer, in which the users exchange data;
and release, in which the connection is discarded.
Datagram
A self-contained unit of data, with an associated destination IP address and
upper-layer protocol number, that is used in series to transmit a whole body of
data from one device to another to the correct service layer protocol.
Device
A network element.
Exterior Gateway Protocol (EGP)
An older protocol used by gateways in a two-level internet (autonomous internet
sites are connected to the Internet through core gateways). All traffic received
from or transmitted between internet sites passes through the core gateway(s).