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Glossary
G-21
hash message authentication code (HMAC) A type of checksum calculation used
by IP Security (IPSec) systems to ensure that data is not modified during
transmission over the network.
hierarchical name space A namespace, such as the Domain Name System (DNS)
and Active Directory service, that has a tiered structure that allows names
and
objects to be nested inside each other.
histogram A chart consisting of horizontal or vertical bars, the widths or
heights of
which represent the values of certain data.
hop
A unit of measurement used to quantify the length of a route between two
computers on an internetwork, indicated by the number of routers that
packets
must pass through to reach the destination end system. For example, if
packets
must be forwarded by four routers in their journey from end system to end
system,
the destination is said to be four hops from the source. Distance vector
routing
protocols like the Routing Information Protocol (RIP) use the number of hops
as
a way to compare the relative efficiency of routes.
horizontal network A network that connects multiple computers and is itself
connected to a backbone network. See backbone network.
host name The name of a device on a network. For a device on a Microsoft
Windows Server 2003 network, this can be the same as the computer name.
Hosts An ASCII text file used by Transmission Control Protocol/Internet
Protocol
(TCP/IP) computers to resolve host names into IP addresses. The Hosts file
is a
simple list of the host names used by TCP/IP computers and their equivalent
IP
addresses. When a user or an application refers to a computer using a host
name,
the TCP/IP client looks it up in the Hosts file to determine its IP address.
The
Hosts file was the original name resolution method for what later became the
Internet until the number of computers on the network grew too large to
manage
using this technique. Eventually, the Domain Name System (DNS) was created
to
perform the same function in a more efficient and manageable way. TCP/IP
computers still have the ability to use a Hosts file for name resolution,
but because
the names and addresses of each computer must be added manually, this method
is rarely used today.
hotfix A small Microsoft software update, released between service packs,
that is
designed to address a specific issue.
HTTP
See Hypertext Transfer Protocol (HTTP).
hub
A hardware component to which cables running from computers and other
devices are connected, joining all the devices into a network. In most
cases, the
term hub refers to an Ethernet multiport repeater, a device that amplifies
the
signals received from each connected device and forwards them to all the
other
devices simultaneously. See also multiport repeater.
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