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Forwarding Storm

A forwarding storm is network congestion caused by a looped network. When a bridge or switch receives a frame for which it doesn't have the destination MAC address in its table, or when it receives broadcast or multicast traffic, it floods the frame out every other port. In a looped network, this can result in the same data being received on multiple ports and being forwarded again and again. A forwarding storm can block all other network traffic and bring a network to a complete stand still. To prevent loops, bridges and switches can use the Spanning Tree Protocol, or STP.

Forwarding Table/MAC Address Table

A forwarding table or MAC address table is where a switch stores address and location information for all devices connected directly to its ports.


Fragmentation is the process of breaking an IP packet into smaller chunks or fragments when transmitted over data link technology with a smaller MTU.


A frame is one unit of data encapsulated at Layer 2, or the Data Link Layer. Each frame is divided into three parts: the header, the data, and the trailer. The frame header contains the data's destination and source Layer 2 addresses. It also indicates which Layer 3 protocol should be used to process the data on the receiving computer. (In the examples in this course, the IP protocol is used.) The frame trailer is a checksum, which is used to verify data integrity.

Frame Relay

Frame Relay is a Layer 2 WAN protocol used to interconnect multiple locations or LANs over a single communications line-such as a T1/T3 or E1/E3 line. With this single line, customers can communicate with multiple sites using virtual circuits. Frame Relay is a cost-effective alternative to dedicated point-to-point connections and can be used to build a Layer 2 Virtual Private Network.

Frame Check Sequence (FCS)

Frame Check Sequence (FCS) is a 2-byte or 4-byte checksum computed over the frame to provide basic protection against errors in transmission.


File Transfer Protocol (FTP) is a network protocol used to transfer data from one computer to another through a network, such as over the Internet.

Forwarding Equivalency Class (FEC)

A Forwarding Equivalency Class (FEC) is a group of packets that the provider's MPLS domain forwards and treats the same.

Full Duplex

Full-duplex data transmission allows for communication in two directions at the same time. That is, a device can receive and transmit data simultaneously. This functionality is similar to using a telephone where you can talk and listen at the same time.

Global Routing Prefix

In an IPv6 address, the global routing prefix is the first part of the network prefix, and is followed by a subnet ID. The global routing prefix is typically assigned by an ISP or regional address allocation entity.

Greenfield Preamble

A Greenfield preamble is used in 802.11n to indicate to communication devices the beginning of a new frame. In an environment that has no legacy devices, using the Greenfield preamble can increase throughput because it is shorter than a legacy preamble. However, in mixed environments, both preambles must be used.

Half Duplex

Half-duplex data transmission allows for communication in two directions, but only in one direction at a time. That is, a device cannot receive and transmit data simultaneously. This functionality is simlar to using a walkie talkie where if you are speaking, you cannot hear the person on the other end.

Handshake or Three-way Handshake

A handshake or three-way handshake is the three-step process two devices go through to establish a connection before they can communicate.

Hardware Ports

SeeNetwork Interface Card (NIC)


The hexadecimal numbering system is a base 16 numbering system, in contrast to the commonly used base 10 decimal numbering system. In the hexadecimal numbering system, the digits 0 through 9 are used the same as they are in decimal numbering. After that, the letters A through F are used to represent the decimal number 10 through 15. For instance, decimal 15 is hexadecimal F and decimal 16 is hexadecimal 10.

Hidden Node Problem

In wireless networks, the hidden node problem occurs when there are two clients on either side of an access point which are within range of the access point, but which can't actually hear each other's transmissions. In this situation both clients may simultaneously transmit data, resulting in collisions at the access point, which is unable to interpret either signal.

High Level Data Link Control (HDLC)

High Level Data Link Control (HDLC) is a Layer 2 protocol developed by the International Organization for Standardization for use on point-to-point communication links. HDLC was a precursor to PPP.

Hypertext Markup Language (HTML)

Hypertext Markup Language (HTML) is the language the Web browser and Web server use to create and display Web pages.


The Hypertext Transfer Protocol (HTTP) is a communications protocol for the transfer of information on the Internet and the World Wide Web.

Host Number

The host number is the second part of a Layer 3 address. (The network number is the first part.) The host number portion of the Layer 3 address identifies the unique device on a network. Both the network number and host number are assigned by a network administrator.


A hub is a Layer 1 device that takes a signal that it receives from one connected device and passes it along or repeats it to all other connected devices. A hub allows each device to use its own twisted-pair cable to connect to a port on the hub. If a cable fails, it will impact only one device, and if one device is causing trouble on the network, that individual device can easily be unplugged. A hub is not an intelligent network device. It does not look at the MAC addresses or data in the Ethernet frame and does not perform any type of filtering or routing of the data. It is simply a junction that joins all the different devices together. Even though each device has its own cable connecting it to the hub, access to the network still operates by CSMA/CD, and collisions can occur on the shared bus inside the hub.

Indirect Routing

Indirect Routing is communication between devices with the help of a router.


The Internet Corporation for Assigned Names and Numbers (ICANN) is the current organization that is responsible for all IP address assignment and domain name registration. ICANN assumed control of IANA in 1998.

Interface ID

In IPv6, the interface ID is the host portion of the address that follows the network prefix.

Internet Assigned Numbers Authority (IANA)

The Internet Assigned Numbers Authority (IANA) is the organization that centrally managed the global IP address space.

The Internet Message Access Protocol (IMAP)

The Internet Message Access Protocol (IMAP) is a protocol for retrieving email from a server.

Internet Layer

In the TCP/IP Reference Model, the Internet Layer receives a segment from the Transport Layer, adds a header to it to create a packet, and sends the packet to the Network Access Layer. The Internet Layer is responsible for delivering the packet to the destination computer. If there is more than one route to the destination computer, the Internet Layer chooses the best path for the packet to take. The Internet Layer treats each packet independently, so it is possible that two packets from the same transmission might take different paths to arrive at the destination computer. This is the same as the OSI Reference Model Network Layer and the Five-Layer Model Network Layer.

Internet Protocol (IP)

The Internet Protocol (IP) is the most common Layer 3 protocol and is used within the Internet to route packets to their final destination. IP provides connectionless, best-effort delivery of packets through a network and fragmentation and reassembly of packets going across Layer 2 networks with different maximum transmission units (MTUs). Each computer or host has at least one IP address that uniquely identifies it from all other computers on the Internet. The IP addressing scheme is fundamental to the process of routing packets through a network.

IP Control Protocol (IPCP)

IP Control Protocol (IPCP) is the PPP Network Control Protocol used to configure, enable, and disable the IP protocol on both ends of the point-to-point link.

IP Interfaces

IP interfaces are Layer 3 software interfaces or ports that work with the hardware interface or port.

Internet Service Provider (ISP)

An Internet Service Provider (ISP) is a company or organization that has the network equipment and communication lines to provide connectivity to the Internet for customers.

Internet Stream Protocol

Internet Stream Protocol, which was considered by some to be IP Version 5, was developed between IPv4 and IPv6 and never made it past experimental stages.


An intranet is a private, internal network that uses the sameIP-based protocols used in the Internet. Intranets often use IP addresses from the private IP address space.

IP Version 4 (IPv4)

IP Version 4 (IPv4) is currently the most commonly used version of the IP protocol. It consists of a 32-bit address and allows for about 4.3 billion unique addresses.

IP Version 6 (IPv6)

IP Version 6 (IPv6) is the next-generation version of the IP protocol. It consists of a 128-bit address, supports more levels of addressing hierarchy, and provides a much greater supply of addresses than IPv4.

IP Datagram

IP datagram is a synonym for an IP packet, which is the data combined with the IP header.


Keepalives are the periodic messages two devices exchange to maintain a connection.

Label Switching Routers (LSRs)

Label Switching Routers (LSRs) are routers running the MPLS protocol.

Label Edge Router (LER)

A Label Edge Router (LER) is a special type of ingress or egress LSR that is responsible for assigning the appropriate MPLS label to a packet.

Label Switched Paths (LSPs)

Label Switched Paths (LSPs) are the unidirectional paths used to connect each location in an MPLS network.

Layer 2 Address

When sending data on a network, Layer 2 addresses identify the stops made along the way, and not the communicating computers or endpoints. Layer 2 addresses change with each stop along the route to the final destination. A Layer 2 address is also known as a MAC address, a hardware address, or a physical address.

Layer 3 Address

A Layer 3 address identifies a unique destination. A network administrator assigns each computer on a network a unique Layer 3 address. The destination and source Layer 3 addresses identify the communicating computers or end points. A Layer 3 address is also known as an IP address or a logical address.

Layer 3 MPLS VPNs or IP VPNs

Layer 3 MPLS VPNs or IP VPNs define a way for providers to use their IP backbone to provide IP-based VPN services regardless of what Layer 2 technology is in use. Layer 3 MPLS VPNS use label switched paths to interconnect different customer locations and keep traffic separated. Customer edge routers exchange IP routing information with the provider's routers where it is stored in a virtual router and forwarding table. Provider Edge routers exchange routes with other provider edge routers within the VPN. When IP packets enter the MPLS network, the provider edge router determines which LSP to use to based on the routing information in the virtual router and forwarding table .

Leased Line

A leased line is a pre-established communications path between two locations with a fixed amount of bandwidth-whether or not it is used. Leased lines are owned by the service provider and not by the customer.

Link Control Protocol (LCP)

Link Control Protocol (LCP) packets are the packets exchanged between two devices during the PPP connection establishment process.


Any group of computers on a single, geographically limited network is a called a local area network (LAN). LANs allow users to exchange documents and share resources such as printers or file servers. A LAN can be either wired or wireless, or a combination of both.

Local Loop or Last Mile

Local loop or last mile refers to the copper or fiber cable that connects the CPE to the nearest CO.

Longest Match Routing

Longest Match Routing is also known as longest match route lookup. It is the process a router uses to select the best route in its routing table when multiple routing entries match. The longest match is the entry with the most bits turned on in the network mask.

Loopback Interface

The loopback interface is a virtual software interface, not associated with or connected to any hardware, that a device uses to send a message back to itself. The loopback interface is commonly used for troubleshooting and network testing. The loopback interface's IP address is

MAC Protocol

In networking, a Media Access Control (MAC) protocol controls access to the physical wire or segment. Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as its MAC protocol. Other MAC protocols exist for other technologies.

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