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DCN Quiz 2 · Plain language first, precise definition next

Definitions you can explain

Use the short meaning to get the idea. Open the details for the formal definition, an example and the distinction your professor could test.

Start with layer, protocol, service, encapsulation, then compare IP address, MAC address and port number.

Find a term or an idea

82 definitions

Application

Application layer

The layer where networked programs define the meaning of requests and responses.

Formal meaning, example & common confusion

Formal definition: The top layer of the Internet stack, containing protocols used directly by applications, such as HTTP, DNS, and DHCP.
Example: A browser and web server exchange HTTP messages at the application layer.
Do not confuse it with…: Applications can rely on lower-layer services; they do not need to implement link delivery themselves.

Across layers

ARP

On an IPv4 local network, ARP helps find the link-layer address associated with a nearby IP address.

Formal meaning, example & common confusion

Formal definition: The Address Resolution Protocol resolves an IPv4 protocol address to a link-layer address on the local subnet.
Example: Before sending a frame to a local neighbor, a host may broadcast an ARP request asking who has a particular IPv4 address.
Do not confuse it with…: ARP sits at the L2/L3 boundary: it uses IP information to learn a MAC address, and its broadcast is local to the subnet.

Across layers

Bandwidth

A measure of how much data a link can carry per unit time, often stated in bits per second.

Formal meaning, example & common confusion

Formal definition: The capacity or configured data rate of a communication channel, commonly expressed in b/s or multiples thereof.
Example: A 100 Mb/s link has a higher nominal data rate than a 10 Mb/s link.
Do not confuse it with…: Bandwidth is capacity, not the time a particular packet takes to arrive; that is latency.

Network

Bellman-Ford relation

A shortest-path rule: the best route to a destination can be found by trying each neighbor as the first step.

Formal meaning, example & common confusion

Formal definition: The recurrence D_x(y) = min_v { c(x,v) + D_v(y) }, expressing node x's least cost to y through a neighbor v.
Example: If a destination costs 5 through one neighbor and 3 through another, the router chooses the cheaper total.
Do not confuse it with…: The relation underlies distance-vector updates; it is not the same algorithmic procedure as Dijkstra's.

Physical

Bit

A 0 or 1, the smallest unit of digital information represented in a network signal.

Formal meaning, example & common confusion

Formal definition: A binary digit encoded physically through a medium using a signaling scheme.
Example: A fiber link represents a bit pattern as changes in light.
Do not confuse it with…: Bits are the information; electrical, radio, or optical signals are how those bits are represented physically.

Data link

Bridge

A device that joins LAN segments and forwards frames using link-layer addresses.

Formal meaning, example & common confusion

Formal definition: A data-link forwarding device that connects LAN segments and filters frames based on MAC addresses.
Example: A bridge can join two Ethernet segments into one link-layer network.
Do not confuse it with…: A bridge forwards frames within link-layer scope; a router connects distinct IP networks.

Across layers

Checksum

A calculated value used to detect some accidental changes to transmitted data.

Formal meaning, example & common confusion

Formal definition: A value computed over selected fields or bytes and checked by the receiver for integrity errors.
Example: IPv4 includes a header checksum; Ethernet uses a frame check sequence for error detection.
Do not confuse it with…: A checksum can detect many errors but is not encryption and does not guarantee correction.

Network

CIDR

A way to write an IP address block with its prefix length after a slash.

Formal meaning, example & common confusion

Formal definition: Classless Inter-Domain Routing represents variable-length IP prefixes, such as 203.0.113.0/24.
Example: A /26 has 6 suffix bits and 64 total IPv4 address patterns.
Do not confuse it with…: The number after the slash is prefix bits, not the usable host count.

Across layers

Connectionless

A communication style where each unit can be sent without first creating a lasting transport connection.

Formal meaning, example & common confusion

Formal definition: A service model in which data units are transmitted independently without transport-level connection setup or maintained session state.
Example: UDP sends datagrams without establishing a TCP connection first.
Do not confuse it with…: Connectionless communication can still use source and destination addresses, and applications can add their own state.

Network

Control plane

The part of networking that figures out or manages where traffic should go.

Formal meaning, example & common confusion

Formal definition: Functions that determine routes, maintain network state, and install forwarding information in devices.
Example: A routing algorithm calculates the next hops that routers use in their data planes.
Do not confuse it with…: Control-plane decisions influence forwarding, but they are not the forwarding of each individual packet.

Network

Data plane

The part of a router that handles packets as they arrive and sends them onward.

Formal meaning, example & common confusion

Formal definition: The forwarding functions that process packets locally according to installed forwarding state.
Example: A router's data plane looks up a destination prefix and selects an output interface.
Do not confuse it with…: The data plane forwards; the control plane computes or manages the rules it uses.

Data link

Data-link layer

The layer that packages data for delivery across one local link and uses link addresses such as MAC addresses.

Formal meaning, example & common confusion

Formal definition: A layer providing framing and link-level delivery over a particular physical medium or local network.
Example: Ethernet delivers a frame from a host to its local router on a LAN.
Do not confuse it with…: A MAC destination is usually replaced at each routed hop, while the IP destination can remain the same end-to-end.

Across layers

Decapsulation

As data moves up the receiving stack, each layer reads and removes its own wrapper.

Formal meaning, example & common confusion

Formal definition: The receiving-side process of processing a layer's control information and passing its payload upward.
Example: A receiver checks the frame, removes its link header, then IP processes the datagram inside.
Do not confuse it with…: Decapsulation occurs layer by layer; the application does not normally strip the Ethernet header itself.

Network

Default gateway

The local router a host sends packets to when the destination is outside its own subnet.

Formal meaning, example & common confusion

Formal definition: A host's configured next-hop router used for destinations lacking a more specific local route.
Example: A laptop sends an Internet-bound packet to its Wi-Fi router as the default gateway.
Do not confuse it with…: The gateway is the next local step, not necessarily the final destination or the entire route.

Application

DHCP

A protocol that lets a device automatically obtain an address and other network settings.

Formal meaning, example & common confusion

Formal definition: The Dynamic Host Configuration Protocol configures hosts with parameters such as IP address, subnet mask, gateway, and DNS server.
Example: A laptop joining campus Wi-Fi broadcasts a request and receives a leased address.
Do not confuse it with…: DHCP is an application-layer protocol despite configuring network-layer settings.

Network

Dijkstra's algorithm

A method for finding least-cost paths from one starting router when link costs are known.

Formal meaning, example & common confusion

Formal definition: An iterative shortest-path algorithm that finalizes the least-cost path to the nearest unsettled node at each step, for nonnegative edge costs.
Example: A link-state router can run Dijkstra over its topology map to build next-hop choices.
Do not confuse it with…: Dijkstra is a path-computation algorithm, not a message protocol by itself.

Network

Directed broadcast

An IP destination that targets all hosts on a particular subnet by setting its suffix bits to 1.

Formal meaning, example & common confusion

Formal definition: The subnet-directed broadcast address is the address with the subnet prefix and an all-ones host portion.
Example: 192.168.1.127 is the directed broadcast for 192.168.1.64/26.
Do not confuse it with…: It differs from 255.255.255.255, the limited broadcast; routers may forward directed broadcasts subject to policy.

Network

Distance-vector (DV) routing

A routing approach where each router tells neighbors its current estimated costs to destinations.

Formal meaning, example & common confusion

Formal definition: A distributed routing algorithm that updates distance estimates using neighbor information and the Bellman-Ford relation.
Example: A router compares the cost through each neighbor and selects the least-cost next hop.
Do not confuse it with…: DV exchanges destination costs with neighbors rather than distributing a complete topology map.

Application

DNS

The naming system that helps find an IP address or other records associated with a domain name.

Formal meaning, example & common confusion

Formal definition: The Domain Name System is a distributed hierarchical naming database and application protocol.
Example: A resolver looks up the address records for example.com before a browser connects.
Do not confuse it with…: DNS is an application-layer protocol even though its answers help the network locate a destination.

Across layers

Encapsulation

As data moves down the sending stack, each layer wraps it with its own control information.

Formal meaning, example & common confusion

Formal definition: The process by which a protocol adds a header and, in some layers, a trailer to data received from above.
Example: An application message gets a TCP header, then an IP header, then a link header and trailer.
Do not confuse it with…: The lower layer carries the upper layer's whole unit as payload; it does not reinterpret the application meaning.

Data link

Ethernet

A common family of wired LAN technologies that defines frame formats and link behavior.

Formal meaning, example & common confusion

Formal definition: IEEE 802.3 technologies specify physical and data-link operation for Ethernet networks.
Example: A desktop sends an IP datagram inside an Ethernet frame to its local router.
Do not confuse it with…: Ethernet is a link technology; it does not determine the end-to-end IP route by itself.

Physical

Fiber optic cable

A cable that carries information as light through thin glass or plastic strands.

Formal meaning, example & common confusion

Formal definition: An optical transmission medium that propagates modulated light signals between transceivers.
Example: A long-distance network link may use fiber to carry high-rate signals.
Do not confuse it with…: Fiber is a physical medium; the protocols carried over it can still be Ethernet and IP.

Network

Forwarding

The router's local act of choosing where to send one arriving packet next.

Formal meaning, example & common confusion

Formal definition: The data-plane operation that maps an input packet and destination to an outgoing interface or next hop.
Example: A router forwards a datagram out the ISP-facing interface after a table lookup.
Do not confuse it with…: Forwarding handles individual packets; routing computes paths and builds forwarding information.

Network

Forwarding table

A router's lookup list mapping destination networks to outgoing interfaces or next hops.

Formal meaning, example & common confusion

Formal definition: A data-plane table used to select packet forwarding actions, often with longest-prefix matching.
Example: A row for 203.0.113.0/24 may direct packets out a particular interface.
Do not confuse it with…: A forwarding table is used for packet delivery; routing protocols and algorithms create or update its contents.

Network

Fragment offset

A field that says where a fragment's data belongs within the original IP datagram.

Formal meaning, example & common confusion

Formal definition: The IPv4 fragment-offset field locates fragment payload in units of eight bytes relative to the original datagram payload.
Example: A later fragment has a nonzero offset indicating that earlier payload bytes came before it.
Do not confuse it with…: The offset is not a byte count in every representation; IPv4 encodes it in eight-byte units.

Network

Fragmentation

Splitting a large IP datagram into smaller pieces so it can cross a link with a smaller MTU.

Formal meaning, example & common confusion

Formal definition: The division of an IP datagram into fragments that are reassembled by the destination; IPv4 and IPv6 handle router fragmentation differently.
Example: An IPv4 datagram may be fragmented when it is too large for a link and fragmentation is allowed.
Do not confuse it with…: A fragment is not a complete application message; all required pieces must arrive for reassembly.

Data link

Frame

A link-layer unit sent across one local network link, with link control information around its payload.

Formal meaning, example & common confusion

Formal definition: A data-link PDU with a link header and commonly a trailer, encapsulating a network-layer packet.
Example: An Ethernet frame can carry an IP datagram to the next router.
Do not confuse it with…: The frame is link-local; the IP datagram inside may continue across many links.

Application

HTTP

A web protocol for requests and responses, such as asking for a page or sending form data.

Formal meaning, example & common confusion

Formal definition: The Hypertext Transfer Protocol defines application messages and semantics for transferring web resources.
Example: A browser sends an HTTP request for a page and receives an HTTP response.
Do not confuse it with…: HTTPS commonly means HTTP protected by TLS, but HTTP/3 uses QUIC rather than TCP; HTTP is not tied to one transport in every version.

Physical

Hub

A simple multiport device that repeats incoming signal energy out its other ports.

Formal meaning, example & common confusion

Formal definition: A physical-layer repeater with multiple ports; it does not inspect destination MAC addresses.
Example: A hub sends a received bit pattern to every connected station except the incoming port.
Do not confuse it with…: Unlike a switch, a hub does not learn addresses or filter frames.

Network

ICMP

A network-layer companion protocol used for IP error reports and diagnostic messages.

Formal meaning, example & common confusion

Formal definition: The Internet Control Message Protocol carries control and error messages associated with IP delivery.
Example: A ping test commonly uses ICMP Echo messages carried inside IP packets.
Do not confuse it with…: ICMP is carried inside IP and operates at the network layer; it is not a transport protocol with application ports.

Across layers

Interface

The word has two common meanings: a connection point on a device, or a boundary where layers exchange data.

Formal meaning, example & common confusion

Formal definition: A device interface is a network attachment point; a layer interface specifies how adjacent layers access each other's services.
Example: A router has several network interfaces; the transport-to-network interface passes a segment for delivery.
Do not confuse it with…: Do not confuse a router's physical or logical network port with the abstract interface between protocol layers.

Network

IP

The Internet Protocol family that carries datagrams between networks using IP addresses.

Formal meaning, example & common confusion

Formal definition: A network-layer protocol providing connectionless, best-effort datagram delivery and addressing across interconnected networks.
Example: A router examines an IP destination and forwards the datagram toward the next network.
Do not confuse it with…: IP does not provide TCP's reliable ordered byte stream; reliability can be added by a higher layer.

Network

IP address

A network-layer address assigned to an interface so packets can be delivered across networks.

Formal meaning, example & common confusion

Formal definition: A logical address used by IP for source and destination identification and forwarding; the address format depends on IP version.
Example: A router has a different IP address on each of its network interfaces.
Do not confuse it with…: The IP address is not a port number or MAC address: the three identify different scopes and jobs.

Network

IP datagram

A network-layer packet with IP addresses and data to deliver across networks.

Formal meaning, example & common confusion

Formal definition: An IP packet containing an IP header and payload, forwarded toward a destination address.
Example: An IP datagram can carry a TCP segment or a UDP datagram.
Do not confuse it with…: Routers forward IP datagrams; each link carries them inside a link-layer frame.

Network

IPv4

The widely used IP version with 32-bit addresses, usually written as four decimal octets.

Formal meaning, example & common confusion

Formal definition: Internet Protocol version 4, whose addresses are 32 bits and whose packets use an IPv4 header.
Example: 192.0.2.8 is an IPv4 address written in dotted-decimal form.
Do not confuse it with…: An IP address belongs to a network interface; it is not a permanent identity for the whole device.

Network

IPv6

A newer IP version with 128-bit addresses, designed in part to provide a much larger address space.

Formal meaning, example & common confusion

Formal definition: Internet Protocol version 6, using 128-bit addresses and an IPv6 packet header.
Example: An IPv6 address is written in hexadecimal groups separated by colons.
Do not confuse it with…: IPv6 is not simply a longer IPv4 address; it has its own packet format and address notation.

Across layers

Latency

The time it takes data to travel from one point to another, including relevant waiting and processing.

Formal meaning, example & common confusion

Formal definition: End-to-end or per-hop delay, commonly composed of transmission, propagation, processing, and queueing delays.
Example: A 30 ms round-trip latency means a request and response take about 30 milliseconds to complete the round trip.
Do not confuse it with…: A link can have high bandwidth and still have high latency, such as a fast long-distance path.

Across layers

Layer

A way to group networking jobs so each group can be explained and changed separately.

Formal meaning, example & common confusion

Formal definition: A conceptual level in a layered network architecture, with defined responsibilities and interactions with adjacent levels.
Example: The network layer handles IP delivery; the link layer moves a frame across one local link.
Do not confuse it with…: A layer is a role grouping, not a physical box or a protocol.

Network

Limited broadcast

The IPv4 destination 255.255.255.255, used to reach the local link before a host has normal IP configuration.

Formal meaning, example & common confusion

Formal definition: An IPv4 local broadcast address whose scope is the directly connected network and which routers do not forward.
Example: A new DHCP client can broadcast a request to 255.255.255.255.
Do not confuse it with…: It is not the all-ones suffix address for one specific subnet; that is a directed broadcast.

Network

Link-state (LS) routing

A routing approach where routers learn network link information and compute paths from a shared map.

Formal meaning, example & common confusion

Formal definition: A class of routing algorithms in which nodes distribute link-state information and calculate least-cost paths, commonly using Dijkstra's algorithm.
Example: A router builds a topology map and runs Dijkstra to find a low-cost route to each destination.
Do not confuse it with…: Link-state routers share link information; distance-vector routers exchange distance estimates with neighbors.

Data link

MAC address

A link-layer address used to deliver a frame on a local link.

Formal meaning, example & common confusion

Formal definition: A Media Access Control address identifies a network interface for data-link delivery within the scope of a link technology.
Example: A host sends a frame to its router's MAC address when the final IP destination is outside the local subnet.
Do not confuse it with…: MAC addresses are used on a local link and can change at each routed hop; IP addresses identify the network-layer source and destination across the path.

Data link

MAC table

A switch's learned list of MAC addresses and the ports where those addresses were seen.

Formal meaning, example & common confusion

Formal definition: A bridge or switch forwarding database mapping link-layer addresses to switch ports.
Example: After seeing a frame from MAC A on port 2, a switch records that location in its MAC table.
Do not confuse it with…: A MAC table maps local frame destinations to ports; a router forwarding table maps IP prefixes to next hops.

Application

Message

Application data ready to be sent using network services.

Formal meaning, example & common confusion

Formal definition: An application-layer unit exchanged by application protocols.
Example: An HTTP request asking a web server for a page is an application message.
Do not confuse it with…: A message may be split across multiple transport units; it is not automatically one packet.

Data link

MTU

The largest IP packet that can fit in one link frame without extra handling.

Formal meaning, example & common confusion

Formal definition: Maximum Transmission Unit: the maximum packet size supported by a link-layer technology or path segment.
Example: If an IP datagram exceeds a link's MTU, it may need fragmentation or another size adjustment, depending on IP version and settings.
Do not confuse it with…: MTU is a size limit in bytes, not a transmission speed.

Across layers

Multicast

Communication from one sender to a selected group of receivers.

Formal meaning, example & common confusion

Formal definition: One-to-many group delivery using an address that identifies a multicast group.
Example: A network video stream may be sent to a multicast group rather than separately to every receiver.
Do not confuse it with…: Multicast is group delivery, while broadcast targets all nodes in a defined scope.

Network

NAT

A router function that rewrites IP address information as packets pass between networks.

Formal meaning, example & common confusion

Formal definition: Network Address Translation maps addresses, often between private internal addresses and a public address, by changing packet headers.
Example: A home router can translate several household devices' private addresses to an ISP-facing address.
Do not confuse it with…: NAT changes address information; it is not the same thing as routing, even though it is commonly done by a router.

Network

Network address

The first address in a conventional subnet block, with all suffix bits set to zero.

Formal meaning, example & common confusion

Formal definition: An address identifying an IP subnet, calculated by ANDing an address with its subnet mask.
Example: The network address for 192.168.1.77/26 is 192.168.1.64.
Do not confuse it with…: It identifies the block; it is usually not assigned as an ordinary interface address.

Network

Network layer

The layer that addresses and moves packets across multiple interconnected networks.

Formal meaning, example & common confusion

Formal definition: The layer responsible for logical addressing, routing, and forwarding of datagrams, primarily through IP in the Internet.
Example: A router reads the destination IP address and chooses a next hop.
Do not confuse it with…: Network layer delivery can cross many links; data-link delivery is for the current link.

Network

Network prefix

The leading bits of an IP address that identify its network block.

Formal meaning, example & common confusion

Formal definition: The fixed-length most-significant portion of an IP address specified by a prefix length or mask.
Example: In 192.168.1.10/24, the first 24 bits form the prefix.
Do not confuse it with…: The prefix length is a count of bits, not a count of decimal address fields.

Network

Next hop

The next router or local destination to receive a packet along its route.

Formal meaning, example & common confusion

Formal definition: The immediate forwarding neighbor selected for a packet toward its destination.
Example: A router may send a datagram to an ISP router as the next hop.
Do not confuse it with…: A next hop is one step in a path; it is not the final destination address.

Data link

NIC

A network interface card or adapter that connects a device to a network medium.

Formal meaning, example & common confusion

Formal definition: A Network Interface Controller implements functions for attaching a host or router to a link, including physical and link-layer operations.
Example: A laptop's Wi-Fi NIC sends and receives radio frames.
Do not confuse it with…: A NIC is hardware or a logical adapter; its MAC address and the interface's IP address belong to different layers.

Across layers

OSI model

A seven-layer reference model used to organize and discuss communication functions.

Formal meaning, example & common confusion

Formal definition: The Open Systems Interconnection model separates communication into physical, data-link, network, transport, session, presentation, and application layers.
Example: Ethernet is commonly associated with data-link functions; IP with network-layer functions.
Do not confuse it with…: OSI is a model for reasoning, not one protocol stack that every Internet application must implement exactly.

Network

PAT

A form of address translation that lets many inside connections share one outside IP address using transport port numbers.

Formal meaning, example & common confusion

Formal definition: Port Address Translation maps address-and-port combinations to distinguish concurrent flows through a translated address.
Example: A home router can map two devices' separate TCP connections to one public IP using different source ports.
Do not confuse it with…: PAT uses logical transport port numbers, not physical ports on the router.

Across layers

Payload

The data carried inside a protocol unit, excluding that unit's own control fields.

Formal meaning, example & common confusion

Formal definition: The encapsulated content delivered to a protocol by the layer above it.
Example: For an IP datagram, the transport segment is the payload.
Do not confuse it with…: The same bytes can be called payload at one layer and a complete unit at the layer that created them.

Physical

Physical layer

The layer concerned with sending encoded bits as electrical, radio, or optical signals over a medium.

Formal meaning, example & common confusion

Formal definition: The lowest communication layer defining signal representation, timing, connectors, frequencies, and transmission media.
Example: A fiber transceiver turns bits into pulses of light.
Do not confuse it with…: The physical layer carries signals; it does not interpret IP addresses or choose routes.

Network

Poisoned reverse

A distance-vector technique that tells a neighbor a route through that neighbor is unreachable, helping prevent some routing loops.

Formal meaning, example & common confusion

Formal definition: A loop-mitigation rule in which a router advertises an infinite metric for a destination back to the neighbor from which that route was learned.
Example: If B reaches X through A, B advertises X as unreachable to A.
Do not confuse it with…: Poisoned reverse can prevent two-node loops, but it does not solve every possible routing loop.

Transport

Port number

A number that helps the transport layer deliver data to the right application process on a device.

Formal meaning, example & common confusion

Formal definition: A transport-layer endpoint identifier used with an IP address to demultiplex traffic to a process or service.
Example: A client sends a DNS query to the server's UDP port 53.
Do not confuse it with…: A port number is a logical software endpoint, not a physical socket or cable connector.

Across layers

Protocol

Shared rules that let networked systems exchange information in a particular way.

Formal meaning, example & common confusion

Formal definition: A specification of message formats, meaning, ordering, and actions taken when messages are sent or received.
Example: TCP defines how endpoints establish a connection and deliver an ordered byte stream.
Do not confuse it with…: A protocol is the rule set; a layer is where a responsibility is grouped, and a service is what a layer offers upward.

Across layers

Protocol data unit (PDU)

The name for the complete unit a layer sends, including its data and control fields.

Formal meaning, example & common confusion

Formal definition: A layer-specific unit consisting of protocol control information and payload.
Example: A TCP segment is the transport-layer PDU; an IP datagram is the network-layer PDU.
Do not confuse it with…: PDU is a general term; names such as segment, datagram, and frame identify particular layers' units.

Transport

Reliable delivery

A service that detects missing data and retries or otherwise ensures the receiver gets an ordered stream or confirmed data.

Formal meaning, example & common confusion

Formal definition: A transport property implemented through mechanisms such as acknowledgments, sequence numbers, retransmission, and duplicate suppression.
Example: TCP retransmits data when acknowledgments indicate that bytes did not arrive.
Do not confuse it with…: Reliable transport does not mean the IP network never loses packets; it means the endpoints handle loss for the application.

Physical

Repeater

A physical-layer device that regenerates a signal so it can travel farther or more reliably.

Formal meaning, example & common confusion

Formal definition: A device that receives and retransmits a signal without interpreting link or network addresses.
Example: A repeater restores a weakened bit signal on a long cable run.
Do not confuse it with…: A repeater does not choose a route or filter frames by MAC address.

Network

Router

A device that connects IP networks and forwards datagrams toward their destinations.

Formal meaning, example & common confusion

Formal definition: A network-layer packet switch that selects an outgoing interface and next hop using forwarding state.
Example: A home router forwards traffic between the household subnet and the ISP network.
Do not confuse it with…: A router has multiple interfaces, usually in different subnets; it is not one endpoint with one address.

Network

Routing

The process of determining paths through a network and deciding which directions destinations should use.

Formal meaning, example & common confusion

Formal definition: The control-plane computation and distribution of reachability or path information used to construct forwarding state.
Example: Routers run a routing protocol to learn which next hop reaches a remote network.
Do not confuse it with…: Routing plans paths; forwarding applies the resulting choice packet by packet.

Network

SDN

Software-defined networking separates network-wide control decisions from packet forwarding devices.

Formal meaning, example & common confusion

Formal definition: An architecture in which control-plane logic can be logically centralized while data-plane devices execute installed forwarding rules.
Example: A controller computes routes and installs forwarding entries in switches or routers.
Do not confuse it with…: Logically centralized control can still be implemented across multiple machines; it does not mean one physical controller must do everything.

Across layers

Service

A useful capability one layer offers to the layer above it.

Formal meaning, example & common confusion

Formal definition: An abstraction describing operations and guarantees available at a layer boundary, independent of the internal implementation.
Example: Transport offers applications ways to exchange data between process endpoints.
Do not confuse it with…: Service describes what is offered; protocol describes rules used between peer systems.

Physical

Signal

A physical change, such as voltage, radio energy, or light, used to carry bits.

Formal meaning, example & common confusion

Formal definition: A measurable physical waveform or state that encodes information over a communication medium.
Example: A Wi-Fi radio signal carries encoded bit patterns through the air.
Do not confuse it with…: The signal is not itself a packet or an address; higher layers give the transmitted bits structure.

Transport

Socket

A software endpoint an application uses to send or receive network data.

Formal meaning, example & common confusion

Formal definition: An operating-system programming abstraction associated with a transport protocol and local/remote endpoint identifiers.
Example: A web server listens on a socket associated with its IP address and port 443.
Do not confuse it with…: A network socket is software; it is not a physical wall socket, cable jack, or router port.

Network

Subnet

A group of interfaces that share an IP prefix and can communicate over the same local network segment.

Formal meaning, example & common confusion

Formal definition: An IP network segment whose interfaces share a subnet prefix; routers connect distinct subnets.
Example: Several computers and one router interface can share 192.168.1.0/24.
Do not confuse it with…: A subnet is a network segment or address block, not necessarily an entire building or one device.

Network

Subnet mask

A 32-bit IPv4 value with 1s for prefix bits and 0s for suffix bits.

Formal meaning, example & common confusion

Formal definition: A bit mask that identifies the network prefix in an IPv4 address; /24 corresponds to 255.255.255.0.
Example: ANDing 192.168.1.77 with 255.255.255.192 gives network address 192.168.1.64.
Do not confuse it with…: A mask marks the boundary; it is not itself an IP address assigned to a host.

Data link

Switch

A LAN device that learns link addresses and sends frames toward the appropriate local port.

Formal meaning, example & common confusion

Formal definition: A link-layer switch forwards frames based on destination MAC addresses and a learned forwarding table.
Example: A switch forwards a frame from one office computer's port toward another computer's port.
Do not confuse it with…: A typical LAN switch forwards frames within a link network; a router forwards IP datagrams between networks.

Transport

TCP

A transport protocol that provides applications with a reliable, ordered byte stream between endpoints.

Formal meaning, example & common confusion

Formal definition: The Transmission Control Protocol is connection-oriented and uses acknowledgments, sequence numbers, and retransmissions to provide reliable ordered delivery.
Example: A web application using HTTP/1.1 can use TCP to deliver its bytes in order.
Do not confuse it with…: TCP reliability applies between endpoints; it does not mean every packet always arrives or that the network layer itself is reliable.

Transport

TCP segment

A TCP transport unit containing application bytes and TCP control fields.

Formal meaning, example & common confusion

Formal definition: A transport-layer PDU with a TCP header and a portion of the connection's byte stream.
Example: A TCP segment carries source and destination port numbers plus sequence information.
Do not confuse it with…: A segment is carried inside an IP datagram; it is not the same thing as an Ethernet frame.

Across layers

TCP/IP model

The layered architecture associated with the Internet protocol suite.

Formal meaning, example & common confusion

Formal definition: A practical model commonly grouping Internet protocols into application, transport, network, link, and physical functions, though diagrams vary in grouping.
Example: HTTP, TCP, IP, and Ethernet illustrate protocols at different levels of an Internet stack.
Do not confuse it with…: TCP/IP names a protocol suite and model; TCP itself is only one transport protocol within it.

Across layers

TLS

A security protocol that can encrypt a connection and help verify who the peer is.

Formal meaning, example & common confusion

Formal definition: Transport Layer Security protects application communication with encryption, integrity checks, and authentication mechanisms, typically used above a transport such as TCP.
Example: HTTPS commonly uses HTTP over TLS over TCP; HTTP/3 instead runs over QUIC, which uses UDP.
Do not confuse it with…: TLS is not the same as TCP reliability, and HTTPS layering depends on the HTTP version.

Across layers

Trailer

Control information placed after carried data, often by a link protocol.

Formal meaning, example & common confusion

Formal definition: A protocol-specific field sequence appended to a protocol data unit.
Example: An Ethernet frame ends with a frame check sequence trailer.
Do not confuse it with…: Not every layer or protocol has a trailer.

Transport

Transport layer

The layer that carries data between application processes on end devices.

Formal meaning, example & common confusion

Formal definition: The end-to-end layer providing process-to-process communication services, including TCP and UDP.
Example: A port number lets a host deliver an arriving segment to the right application process.
Do not confuse it with…: Transport uses ports; network-layer IP addresses get packets between interfaces and networks.

Network

TTL

A field that limits how many router hops an IPv4 packet can take before being discarded.

Formal meaning, example & common confusion

Formal definition: Time To Live is decremented by routers; in practice it bounds packet lifetime by hop count and prevents indefinite looping.
Example: Traceroute uses changing TTL values to reveal successive routers on a path.
Do not confuse it with…: TTL is not a precise wall-clock timer in normal forwarding; think hop limit.

Transport

UDP

A lightweight transport protocol that sends individual datagrams without setting up a TCP-style connection.

Formal meaning, example & common confusion

Formal definition: The User Datagram Protocol provides best-effort message delivery with port-based process addressing and minimal transport control.
Example: A DNS lookup may use UDP for a short query and response.
Do not confuse it with…: Connectionless does not mean addressless: UDP still uses IP addresses and port numbers.

Transport

UDP datagram

A short transport unit sent by UDP with source and destination ports and application data.

Formal meaning, example & common confusion

Formal definition: A UDP message with a header and payload, delivered without TCP's connection setup or ordered-reliability service.
Example: A DNS query commonly travels in a UDP datagram.
Do not confuse it with…: UDP datagram and IP datagram are different units at different layers.

Across layers

Unicast

Communication from one sender to one intended receiver.

Formal meaning, example & common confusion

Formal definition: One-to-one delivery using a destination address identifying a single endpoint or interface.
Example: A browser sends a request to one web server using unicast IP delivery.
Do not confuse it with…: Unicast describes delivery pattern, not a particular protocol or layer.

Data link

Wi-Fi

A family of wireless LAN technologies that connects nearby devices using radio.

Formal meaning, example & common confusion

Formal definition: Wi-Fi is based on IEEE 802.11 standards, including physical radio and data-link functions.
Example: A phone joins a Wi-Fi access point and sends local link frames over radio.
Do not confuse it with…: Wi-Fi is not the Internet itself; it is one possible local link carrying IP traffic.