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Address Resolution Protocol. is an internet protocol used to map an IP address to a MAC address. ARP is a required TCP/IP standard defined in RFC 826. ARP resolves IP addresses used by TCP/IP-based software to Media Access Control addresses used by LAN hardware. ARP provides the following protocol services to hosts located on the same physical network.

Access List


A list kept by routers to control access to or from the router for a number of services (for example, to prevent packets with a certain IP address from leaving a particular interface on the router).





The speed at which a communications system can transfer data, usually measured in bits per second.

Broadcast Address

A special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all ones



Short for Bits Per Second. In data communications, bits per second (abbreviated bps) is a common measure of data speed for computer modem and transmission carriers. As the term implies, the speed in bps is equal to the number of bits transmitted or received each second.



Bottlenecks refers to the delay in transmission of data through the circuits of a computer's microprocessor or over a TCP/IP network. The delay typically occurs when a system's bandwidth cannot support the amount of information being relayed at the speed it is being processed. There are, however, many factors that can create a bottleneck in a system.

Bottlenecks affect network performance by slowing down the flow of information transmitted across networks. TCP/IP connections were originally designed to transmit only text files, and the proliferation of bandwidth-intensive transmissions such as high-resolution graphics has caused bottlenecks in the process; therefore, the data moves more slowly across networks.


Broadcast Storm


An undesirable network event in which many broadcasts are sent simultaneously across all network segments. A broadcast storm uses substantial network bandwidth and, typically, causes network time-outs. Broadcast storms can usually be prevented by carefully configuring a network to block illegal broadcast messages.



A storage area used for handling data in transit. Buffers are used in internetworking to compensate for differences in processing speed between network devices. Bursts of data can be stored in buffers until they can be handled by slower processing devices. Sometimes referred to as a packet buffer.




A byte is a unit of data that is eight binary digits long. A byte is the unit most computers use to represent a character such as a letter, number, or typographic symbol (for example, "g", "5", or "?")




Short for Cisco Discovery Protocol. CDP is a  media- and protocol-independent device-discovery protocol that runs on all Cisco-manufactured equipment including routers, access servers, bridges, and switches. Using CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN or on the remote side of a WAN. The devices do not need to have any network layer protocols configured in order to use CDP, although if these addresses are configured CDP will discover them.. Each device configured for CDP sends out periodic messages to a MAC layer multicast address. these advertisements include information about the capabilities and software version of the advertising platform. This gives you an easy way to see other Cisco devices on your network without having to figure out which devices are Cisco by the vendor code embedded in the Media Access Control address.


Configuration Register

All Cisco Router have a 16-bit configuration register, which is stored in a special memory location in NVRAM. This register controls a number of function, some of which are listed below:

The configuration register boot field is the portion of the configuration register that determines whether the router loads an IOS image, and if so where to get this image from. The least significant four bits, bits 0 through 3, of the configuration register make up the boot field.

If the boot field value is 0x0 ( all four bits set to zeros), the router will enter ROM monitor mode.
If the boot field value is 0x1 (binary 0001), the router will boot from the image in the ROM.
If the boot field value is 0x2 through 0xF ( binary 0010 through 1111) the router will follow the normal boot sequence and will look for boot system commands in the configuration file in NVRAM.


Community String

The Community String is a  "password" that allows either Read Only (RO) or Read/Write (RW) control.






Short for Dynamic Host Control Protocol. An effective way to dynamically assign and reuse a fixed number of IP addresses when there are more devices on the network than addresses available. A DHCP server dynamically assigns IP addresses to devices requesting them. These address assignments expire after a time specified by the network manager. The DHCP server then reassigns these addresses to other devices as needed. DHCP is an extension to BOOTP in which the address assignments are static.



Short for Domain Name System (or Service). The Domain Name System (DNS) is a distributed Internet directory service. DNS is used mostly to translate between domain names and IP addresses. Most Internet services rely on DNS to work, and if DNS fails, web sites cannot be located and email delivery stalls.

DNS Server

The DNS system consists of three components: DNS data (called resource records), servers (called name servers) and Internet protocols for fetching data from the servers.

DNS Name server is a server that runs DNS server programs containing name-to-IP address mappings, IP address-to-name mappings, information about the domain tree structure, and other information. DNS servers also attempt to resolve client queries.

The billions of resource records in the DNS are split into millions of files called zones. Zones are kept on authoritative servers distributed all over the Internet, which answer queries based on the resource records stored in the zones they have copies of. Caching servers ask other servers for information and cache any replies. Most name servers are authoritative for some zones and perform a caching function for all other DNS information. Large name servers are often authoritative for tens of thousands of zones, but most name servers are authoritative for just a few zones.


Dword Address

A portion of memory, usually a variable, which has a length of four bytes. The term dword is given to anything which is four bytes in length. Literally, a dword is a "double word."

For example for the IP Address, enter the following keystrokes into the calculator exactly as shown:

206 * 256 + 191 = * 256 + 158 = * 256 + 55

The dword equivalent of the IP address will be the result. In this case, 3468664375.




Errors and Discards

In Performance monitoring Errors and Discards refers to the number of packets that could not be transmitted because of errors which results in discarded packets.




Forward Lookup

Forward DNS (domain name system) Lookup uses an Internet domain name to find an IP address. When you enter the address for a Web site at your browser, the address is transmitted to a nearby router which does a forward DNS lookup in a routing table to locate the IP address. Forward DNS lookup is the more common lookup since most users think in terms of domain names rather than IP addresses.


Flash Memory

A special type of EEPROM (electrically erasable programmable read-only memory) that can be erased and reprogrammed in blocks instead of one byte at a time. Many modern PCs have their BIOS stored on a Flash memory chip so that it can be updated easily if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in modems because it enables the modem manufacturer to support new protocols as they become standardized.






Giants refers to the number of packets that are discarded by the router because they exceed the medium's maximum packet size.




HEX IP Address

A major numbering system used by computers is hexadecimal or Base 16. In this system, the numbers are counted from 0 to 9 then letters A to F before adding another digit. The letter A through F represent decimal numbers 10 through 15 respectively. The below chart indicates the values of the hexadecimal position compared to 16 raised to a power and decimal values It is much easier to work with large numbers using hexadecimal To convert a value from hexadecimal to binary, you merely translate each hexadecimal digit into its 4-bit binary equivalent. Hexadecimal numbers have either and 0x prefix or an h suffix. For example, the hexadecimal number:


Translates into, Using the Binary chart and the below chart for Hex:

0011 1111 0111 1010. values than decimal.























































Gives the order in which the TCP/IP packets progress from machine to machine, called the 'distance' (in hops) from the originating machine.


Host Address

A fully qualified domain name (usually alphabetic) identifying the address of one specific host computer on the Internet. The host address is a subset of the IP address.





Short for Internet Control Message Protocol. ICMP is an extension to the Internet Protocol (IP) defined by RFC 792. ICMP supports packets containing error, control, and informational messages. The PING command, for example, uses ICMP to test an Internet connection.


In Discards


The number of inbound packets which were chosen to be discarded even though no errors had been detected to prevent their being deliverable to a higher-layer protocol. One possible reason for discarding such a packet could be to free up buffer space.




Short for Internetworking Operating System. The Cisco IOS software is the software that runs on the Cisco products, This platform is integral to the interoperations of network devices in a Cisco internetwork. Cisco IOS includes security, access control, authentication, firewall, encryption, and management services. The main purpose of IOS is to boot the Cisco hardware and begin the optimal transport of data across the internetwork.


IP Address

The 32-bit IP address is often depicted as a dot address that is, four groups (or quads) of decimal numbers separated by periods. Here's an example:

Each of the decimal numbers represents a string of eight binary digits. Thus, the above IP address really is this string of 0s and 1s:


Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address). IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here's how the IP address is divided:

          <--Network address--><--Host address-->

                     130.5     .          5.25

If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address.


           <--Network address--><--Subnet address--><--Host address-->

                      130.5              .               5           .      25






Short for kilobit per second. One kilobit per second (Kbps) equals 1000 bits per second (bps). Network performance is best measured in bps, but sometimes numbers are given in bytes per second (Bps). Then, one KBps equals one kilobyte per second, one MBps equals one megabyte per second, and GBps equals one gigabyte per second.





The time delay of data traffic through a network or a switch.



MAC Address

Short for Media Access Control address. MAC Address is a standardized data link layer address that uniquely identifies each device and is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 6bytes long and area also known as a hardware address, MAC layer address, and physical address.




Short for Management Information Base. MIB is a database of network management information that is used and maintained by a network management protocol, such as SNMP or CMIP. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands, usually through a GUI network management system. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.




The MIB file can contain one or more MIB modules.

Following are the macros defined in the SMIv1 and SMIv2.



The following macro definition is defined only in SMIv1.


The following macro definitions are defined only in SMIv2.












A netmask is a 32-bit mask used to divide an IP address into subnets and specify the networks available hosts. In a netmask two bits are always automatically assigned. For example in, "0" is the assigned network address and in, "255" is the assigned broadcast address. The 0 and 255 are always assigned and cannot be used.



Network Classes


Class A Network - binary address starts with 0, therefore the decimal number can be anywhere from 1 to 126. The first 8 bits (the first octet) identify the network and the remaining 24 bits indicate the host within the network. An example of a Class A IP address is, where "102" identifies the network and "168.212.226" identifies the host on that network.

Class B Network - binary addresses starts with 10, therefore the decimal number can be anywhere from 128 to 191. (The number 127 is reserved for loopback and is used for internal testing on the local machine.) The first 16 bits (the first two octets) identify the network and the remaining 16 bits indicate the host within the network. An example of a Class B IP address is where "168.212" identifies the network and "226.204" identifies the host on that network.

Class C Network - binary addresses starts with 110, therefore the decimal number can be anywhere from 192 to 223. The first 24 bits (the first three octets) identify the network and the remaining 8 bits indicate the host within the network. An example of a Class C IP address is where "200.168.212" identifies the network and "226" identifies the host on that network.

Class D Network - binary addresses starts with 1110, therefore the decimal number can be anywhere from 224 to 239. Class D networks are used to support multicasting.

Class E Network -- binary addresses start with 1111, therefore the decimal number can be anywhere from 240 to 255. Class E networks are used for experimentation. They have never been documented or utilized in a standard way.



Short for Network Interface Card. A network interface card (NIC) is a computer circuit board or card that is installed in a computer so that it can be connected to a network. Personal computers and workstations on a local area network (LAN) typically contain a network interface card specifically designed for the LAN transmission technology, such as Ethernet or Token Ring. Network interface cards provide a dedicated, full-time connection to a network.


Short for nonvolatile RAM. NVRAM is a special memory that does not lose its information when a router is powered off. It stores the system's startup configuration file and the virtual configuration register.





Octet Flow

In  network monitoring the term Octet flow refers to the number of octets transmitted over the interface.



Short for Object Identifier. The OID is a long numeric tag, used to distinguish each variable uniquely in the MIB and in SNMP messages.


Out  Discards

The number of outbound packets which were chosen to be discarded even though no errors had been detected to prevent their being transmitted. One possible reason for discarding such a packet could be to free up buffer space.



Packet Flow

A flow is a set of packets passing an observation point in the network during a certain time interval. All packets belonging to a particular flow have a set of common properties derived from the data contained in the packet and from the packet treatment at the observation point.






Quality of Service standards seek to maximize the use of available network bandwidth by prioritizing time-sensitive traffic.






Short for random-access memory. RAM is a volatile memory that can be read and written by a computer.


Received Errors


The number of inbound packets that contained errors preventing them from being deliverable to a

higher-layer protocol.


Reverse Lookup

Reverse DNS lookup uses an Internet IP address to find a domain name.


Round Trip Time

The Round Trip time refers to the timestamp placed by Ping in each packet, which is echoed back to calculate how long each packet exchange took.



A router is an Intermediate System (IS) which operates at the network layer of the OSI reference model. Routers may be used to connect two or more IP networks, or an IP network to an internet connection.

A router consists of a computer with at least two network interface cards supporting the IP protocol. The router receives packets from each interface via a network interface and forwards the received packets to an appropriate output network interface.

A router introduces delay (latency) as it processes the packets it receives. The total delay observed is the sum of many components including:

The router queue of packets waiting to be sent also introduces a potential cause of packet loss. Since the router has a finite amount of buffer memory to hold the queue, a router which receives packets at too high a rate may experience a full queue. In this case, the router has no other option than to simply discard excess packets. If required, these may later be re-transmitted by a transport protocol.




Short for read-only memory. ROM is the non volatile memory that can be read , but not written , by the computer. The image in ROM is the image the router first uses when it is powered up. This image is usually an older and smaller version of IOS without the features of a full IOS version.




Runts refers to the number of packets that are discarded because they are smaller than the medium's minimum packet size.




SNMP Agent


A management entity consisting of hardware and embedded software which responds to SNMP requests over Ethernet from an SNMP manager




A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnet, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.

The Internet is a collection of networks whose users communicate with each other. Each communication carries the address of the source and destination networks and the particular machine within the network associated with the user or host computer at each end. This address is called the IP address (Internet Protocol address). This 32-bit IP address has two parts: one part identifies the network (with the network number) and the other part identifies the specific machine or host within the network (with the host number). An organization can use some of the bits in the machine or host part of the address to identify a specific subnet. Effectively, the IP address then contains three parts: the network number, the subnet number, and the machine number.

(adapted from

Subnet Mask


A subnet mask is used to determine what subnet an IP address belongs to.



Internet protocol providing e-mail services.






TCP/IP - Stands for "Transmission Control Protocol / Internet Protocol" - TCP/IP is a suite of communications protocols that forms the basis for and defines the Internet.


Transmitted Errors

The number of outbound packets that could not be transmitted because of errors.


Short for Time to Live.
 TTL is a  field in the Internet Protocol (IP) that specifies how many more hops a packet can travel before being discarded or returned.



Timeout is the value to be set in milliseconds to wait for each packet reply before the connection is disconnected as no data is being sent.




A communications protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams (a datagram is the term used to describe data that is packetised for network transport) typically over an IP network. It is used primarily for broadcasting messages over a network. UDP uses the Internet Protocol to get data from one computer or device to another but does not divide a message into sequenced packets nor reassemble it at the other end.



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