Droms R. The DHCP handbook

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host table A file that contains a list of hosts and their link-layer addresses and

DNS names.

hoteling The temporary daily use of offices by staff members.

HTML (Hypertext Markup Language) A system of tags that is used to define

the appearance of a document.

HTTP (Hypertext Transfer Protocol) A protocol that is used to transmit docu-

ments, usually specified in HTML (for example, between World Wide Web servers

and browsers).

ICMP (Internet Control Message Protocol) A protocol in the TCP/IP suite

that provides error reporting and information functions.

IESG (Internet Engineering Steering Group) An organization that guides the

operation of the IETF and sets the status of Internet protocols.

IETF (Internet Engineering Task Force) An organization that is involved in

the evolution of the Internet and development and coordination of new Internet

protocols.

interface identifier A 64-bit value that can be generated from an interface link-

layer address. Interface identifiers are used in IPv6 addresses (see RFC 2343).

internet A set of network segments connected by routers over which hosts use

TCP/IP.

Internet The global collection of public and private networks that use TCP/IP.

Internet Draft A preliminary protocol specification published by the IETF for

review and comment by the Internet community.

internet layer The component of the TCP/IP model that is responsible for end-

to-end delivery of protocol messages between computers.

IP (Internet Protocol) The protocol in the TCP/IP suite that implements the

internet layer.

IP address A 32-bit number that is assigned to a network interface that uniquely

identifies the computer and specifies the network segment to which the computer is

connected.

IP datagram The basic message unit for data delivered by IP.

IPNAT (IP network address translation) See NAT.

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IPng (IP next generation) See IPv6.

IPv6 (IP version 6) A new version of IP that is under development by the IETF.

IRC (Internet Relay Chat) A service for real-time messaging among groups of

network users.

ISC (Internet Software Consortium) An organization that produces reference

implementations of TCP/IP protocols and services, including DHCP and DNS.

ISC DHCP client A freely available implementation of a DHCP client produced

by the ISC.

ISC DHCP relay agent A freely available implementation of a DHCP relay agent

produced by the ISC.

ISC DHCP server A freely available implementation of a DHCP server produced

by the ISC.

LDAP (Lightweight Directory Access Protocol) A protocol that is used to

access directory information from directories based on the X.500 models.

lease The period of time over which a DHCP client can use its assigned IP address.

limited broadcast IP address The IP address used for broadcast by hosts on a

network segment. The limited broadcast IP address is usually 255.255.255.255, but

some legacy devices use 0.0.0.0.

link-layer address The unique identifier for a network interface on a network

segment. A frame usually includes the link-layer addresses of both the destination

and source network interfaces.

Linux An operating system that is modeled after Unix, for which source code is

freely available.

MAC (message authentication code) A value that is computed from the

contents of an authenticated message that cannot be forged. The recipient uses the

MAC to confirm that the contents of the message were not changed.

MAC (Media Access Control) address See link-layer address.

magic cookie or cookie A well-known number. In DHCP, it is a 32-bit number

that identifies the format of the options in the variable-format section of a DHCP

message.

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MD5 (Message Digest Algorithm 5) An algorithm for generating a message

digest that is based on a secret key.

MIB (management information base) A data structure that consists of named

objects shared by SNMP agents and managers. A MIB gives agents and managers a

common naming scheme for values to be managed through SNMP.

mobile device A networked computer or another device that can be moved

among different network segments.

MTU (maximum transmission unit) The largest frame that can be accepted by

a network segment.

multicast A technique for delivering a datagram to multiple destinations by

transmitting a single copy of the datagram across any network segment.

NAT (network address translation) A technique for rewriting the IP addresses

in IP datagrams. It is often used to convert internal, private addresses (for example,

from network 10.0.0.0) to external addresses that can be routed through the

Internet. A NAT box or NAT is a device attached to an internal network that provides

NAT service.

NDS (NetWare Directory Service) A hierarchical naming or directory service

for NetWare.

NetBIOS (Network Basic Input/Output System) A protocol suite originally

developed by IBM and now widely used among computers using the Windows oper-

ating systems.

NetWare A protocol suite and set of network services developed by Novell.

NetWare/IP A version of the NetWare protocol suite that uses IP for datagram

transport.

network analyzer A device that receives all frames from the network segment to

which it is attached and displays the contents of the frames in a variety of formats;

sometimes called a packet sniffer after the name of a widely used commercial network

analyzer.

network segment A distinct physical network. All computers that are connected

to a network segment can transmit directly to each other.

NFS (Network File System) An open-standard protocol originally developed by

Sun Microsystems for the exchange of file contents through a network.

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NIC (network interface card) A device that connects a computer or another

device to a network.

NIS (Network Information Service), NIS+ (Network Information Service+)

A directory service included with the Sun Microsystems Solaris operating system.

NNTP (Network News Transport Protocol) An application protocol for trans-

mission of data, used by the Network News (

netnews) service.

NTP (Network Time Protocol) An application protocol for coordinating inter-

nal clocks among networked computers.

octet An 8-bit data item. Usually synonymous with byte.

option A configuration parameter that is carried in the variable-format section

of a DHCP message. Each option includes an option code that identifies the option, a

length, and the option data.

options or

variable-format

section A section of a DCHP message that carries

DHCP options, whose length is variable and whose contents and format depend on

the option type.

OSPF (Open Shortest Path First) A routing protocol that is based on a link-

state algorithm.

overlay or shared segment See shared network.

packet A generic name for a protocol message. Packet is sometimes used as a

synonym for a link-layer frame, an IP datagram, or a TCP segment.

physical layer The component of the TCP/IP protocol model that delivers data

encoded in a physical representation, such as electrical current, radio waves, or light.

ping

An application that uses ICMP Echo Request/Echo Reply messages to deter-

mine whether two computers can exchange network messages.

PMTU (path maximum transfer unit) The largest IP datagram that can be

transmitted without fragmentation along the path between two computers. In IPv4,

PMTU is used to control the size of TCP segments to avoid fragmentation.

POP (Post Office Protocol) A protocol that is used to access the contents of a

mailbox on a mail server. For example, a desktop computer can use POP to access

mail messages on an organization’s mail server.

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PPP (Point to Point Protocol) A protocol that is used for transmission of IP

datagrams across point-to-point networks such as telephone modem connections.

prefix length The number of bits in the network identifier portion (prefix) of a

IP address. The notation /n indicates a prefix of n bits.

proxy ARP A use of ARP in which a device answers an ARP request for an IP

address that is not assigned to it.

RARP (Reverse Address Resolution Protocol)/DRARP (Dynamic RARP)

Internet protocols. A TCP/IP host uses RARP to find its IP address from a static data-

base based on its link-layer address; DRARP allows a new host to be registered in the

address database automatically.

RAS (remote access server) A device that provides network access to remote

devices connected through, for example, a dialup connection or a tunnel.

relay agent A device that forwards DHCP messages from clients to servers, allow-

ing centrally located DHCP servers to provide DHCP service for clients on several

network segments.

renumbering The process of assigning new IP addresses to all the hosts in a

network.

reservation An entry in a DHCP server database that manually assigns a specific

address to a DHCP client. See static allocation.

reserved addresses IP addresses that are reserved for DHCP clients.

RFC (Request for Comments) A document in the series of documents

published by the RFC Editor that pertains to the Internet and Internet protocols. The

RFCs include protocol specifications, status reports on Internet protocols, and other

reports about Internet-related topics.

RIP (Routing Information Protocol) A routing protocol that is based on a

vector-distance algorithm.

router A network device that has more than one network interface and forwards

IP datagrams between network segments.

router discovery A mechanism (part of ICMP) through which TCP/IP hosts can

find routers connected to the same network segment.

routing table A table that contains the addresses of routers to which IP data-

grams are to be forwarded for delivery.

RR (resource record) An entry in a DNS server that holds DNS data.

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scope In the Microsoft DHCP server, a collection of IP addresses and associated

parameter values to be assigned to DHCP clients. More generally, scope is the range

of influence of an option declaration or variable.

shared network A network segment that has been assigned two or more IP

subnets.

shell script A program that is written in a Unix command interpreter language

such as

sh or csh.

SLP (Service Location Protocol) A protocol for identifying and locating

network services. SLP can provide flexible and dynamic configuration of services also

provided through DHCP options.

SMTP (Simple Mail Transport Protocol) A protocol that is used to deliver e-

mail messages.

source routing A technique in which the source specifies the path for an IP data-

gram to its destination, rather than making routing decisions in routers.

state transition diagram A graphical representation of the states and transi-

tions that describe the behavior of a finite state machine.

static route A fixed routing table entry that is not obtained through a routing

protocol.

static (or fixed) allocation Preconfigured assignment of IP address and configu-

ration parameters to a DHCP client, usually in the DHCP server configuration file.

See also automatic allocation, dynamic allocation.

StreetTalk A directory service that is included with Banyan Vines.

subnet A set of IP addresses that share a common network number, defined either

by the class of the IP address or by a subnet mask.

subnet declaration In the ISC DHCP server configuration file, the definition of

an IP subnet for which the server should manage address assignments.

subnet mask A 32-bit number that identifies which bits of the address make up

the subnet address and which are used as the host address.

subnetting A technique for dividing Class A, B, or C addresses into smaller groups

of IP addresses or subnets that more closely match the addressing requirements for

network segments (see RFC 950, RFC 1878, and RFC 1519).

superscope In the Microsoft DHCP server, a network segment that has been

assigned more than one IP subnet.

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switch A device that provides a direct network connection between pairs of

devices connected to its ports. The switch appears to be a dedicated network

segment to each of the connected devices.

T1 The time at which a DHCP client begins to attempt to extend the lease on its IP

address from the DHCP server that originally assigned the address.

T2 The time at which a DHCP client begins to attempt to extend the lease on its

assigned IP address from any available DHCP server.

TCP (Transmission Control Protocol) A protocol that provides reliable,

connection-oriented delivery of arbitrarily long messages or streams of data.

TCP keepalive A technique for maintaining a TCP connection by periodically

transmitting bytes that are not part of the data in the TCP connection.

TCP/IP (Transmission Control Protocol/Internet Protocol) The suite of

computer communication protocols used in the Internet; the name comes from the

two most important protocols in the suite, TCP and IP. See TCP, IP.

TCP/IP stack An implementation of the TCP/IP suite.

TFTP (Trivial File Transfer Protocol) A simple application protocol that is

built on UDP for copying files between networked computers.

trailers Headers of TCP/IP messages at the end of each message rather than at the

front (see RFC 893).

transaction ID A 32-bit identification number that is used to match DHCP

response messages with DHCP request messages.

TTL (time to live) A field in an IP datagram that limits the lifetime of the data-

gram in an internet. The TTL is initially set to a positive integer and decremented by

routers; if the TTL goes to zero, the datagram is discarded.

UCS (Universal Character Set) A standard that defines encodings for the char-

acters from most of the writing systems in existence today.

UDP (User Datagram Protocol A protocol that provides best-effort, connec-

tionless delivery of discrete messages. UDP is the protocol that is used to carry DHCP

messages.

unicast Delivery of a protocol message to a single destination computer. Unicast

describes the normal delivery of IP datagrams.

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URL (uniform resource locator) A syntactic notation for identifying network

objects that includes an access protocol as well as the location and name of the

object.

UTC (Universal Time Coordinated; formerly Greenwich Mean Time) The

time of day at the Prime Meridian, which does not follow any seasonal adjustments

to local time.

UTF-8 (UCS Transformation Format) An encoding that represents 7-bit ASCII

values in a single byte and then uses multibyte values to represent other characters

from the Unicode Standard.

valid lifetime In IPv6, a parameter associated with an IPv6 address or prefix that

defines the time through which the address or prefix may be used.

variable-format

section The portion of a DHCP message in which options are

carried.

virtual hosting A system that provides for multiple copies of a service—for

example, multiple World Wide Web servers—through a single physical interface by

assigning more than one IP address to the interface.

WINS (Windows Internet Naming System) A protocol that provides dynamic

NetBIOS to IP address name registration and resolution.

X.500 A directory system that was originally part of the OSI protocol suite and is

now widely used with TCP/IP.

X Window System A windowing system that can be used for remote graphical

displays.

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Symbols

-d switch

ISC DHCP client, 367

ISC DHCP server, command-line invocation,

245

-f switch, ISC DHCP server, command-line

invocation, 245

-g switch (silent startup), ISC DHCP client, 367

-lf switch, ISC DHCP server, command-line

invocation, 246

-p switch, ISC DHCP server, command-line

invocation, 245

-pf switch, ISC DHCP server, command-line

invocation, 246

-play switch, ISC DHCP server, command-line

invocation, 246

-q switch, ISC DHCP server, command-line

invocation, 246

-t switch, ISC DHCP server, command-line

invocation, 246

-tf switch, ISC DHCP server, command-line

invocation, 246

/etc/dhcpd.conf file, 234-235

1-minute leases, 327-328

1-year leases, 326-327

A (address) records (DNS), 178, 394

dangling, 408

abandoned leases, recovery of, 200-201

ABANDONED state, IP address binding, 163-164

access control lists

clients, deny unknown statement, 286-288

IP address allocation, 193

versus authentication, 288

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