Half Duplex
The ability to transmit and receive signals in both
directions, but only in one direction at a time
Host
Any device on the network that implements communi-
cation above the Data Link Layer.
Hostname
A synonym for IP domain name, which typically
takes the form host.institution.domain
Hub
A physical layer device, generally used to serve at
the center of a star-topology network. Hubs accept
host, internetworking, and network management
modules; and provide re-timing/repeater, bridging,
and/or network management functionality. Ethernet
2 and IEEE 802.3 terminology, a hub is an Ethernet
multi-port repeater, which sometimes referred to as
a concentrator.
ICMP (Internet Control Message Protocol)
The Internet standard protocol (RFC 792) used to
handle errors and control messages at the IP layer.
ICMP is actually part of the IP protocol.
IGMP (Internet Group Multicast Protocol)
The Internet standard (RFC 1112) extensions to IP
that support IP datagram multicasting to a “host
group.” Membership in a host group is dynamic and
there are no topological restrictions to participating
in a host group. Hosts groups may be either perma-
nent or transient. Host groups are identifi ed by Class
D IP addresses. There are three levels of confor-
mance: 0: no support; 1: support for sending but not
receiving; 2: full support.
IP (Internet Protocol)
Internet Protocol; the most important protocol of the
Internet suite. IP allows information to traverse
multiple networks worldwide.
The network layer protocol (RFCs 760, 791) for the
Internet protocol suite.
Latency
Measure of time, within a forwarding device,
between receipt of the fi rst bit of a packet on input
and the generation of the fi rst bit of the same packet
forwarded on output.
Local Area Network (LAN)
A local area network is generally a private network.
It is under the control of the owner and used by a set
of related individuals and/or workgroups, typically
within a single building or over a group of neighbor-
ing buildings. LANs are a group of computers and
associated devices that share a common communi-
cations line and typically share the resources of a
single processor or server within a small geographic
area (for example, within an offi ce building). Usually,
the server has applications and data storage that are
shared in common by multiple computer users.
A local area network may serve as few as two or
three users (for example, in a home network) or
many as thousands of users (for example, in an
FDDI network).
MAC (Media Access Control)
The protocol level that describes network management
frames sent on the 802.5 token ring. Most MAC frames
are handled transparently by the network adapter.
MAC Address
The unique physical (data link layer) address of an
individual network interface. The standard specifi es
both 16- and 48-bit addresses. The 802.1 standards
committee has defi ned a canonical format for writing
and transmitting a MAC address. It is expressed as
six octets separated by hyphens, each octet repre-
sented as twohexadecimal digits, least-signifi cant bit
fi rst (littleendian). (As a matter of custom, non-
canonical, most-signifi cant bit fi rst representation is
given using colons as the octet separator).
For example, 08-00-20-85-e6-8d is a typical canoni-
cal MAC address. Within the MAC address are a
number of fi elds of interest. The most signifi cant
(fi rst) bit, known as the I/G bit (for Individual or
Group) is 0 if the address represents an individual
station; it is 1 if the address is a multicast (group or
broadcast) address. The second bit, known as the
U/L (for Universal or Local) bit is 0 if the address is
globally administered and registered with the IEEE;
it is 1 if it is a locally administered address.
MAC sublayer
Media Access Control sublayer. As defi ned by the
IEEE, the lower portion of the OSI reference model
data link layer. The MAC sublayer is concerned with
media access issues, such as whether token pass-
ing or contention will be used.
34
Glossary