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Networking Foundations

Glossary

Use this page when you encounter an unfamiliar term. You do not need to memorize every definition.

This glossary provides brief definitions of terms used throughout the guide. Select a linked term for a more detailed reference.

Numbers

Section titled “Numbers”
  • 4-tuple: The source and destination Internet Protocol (IP) addresses and port numbers that uniquely identify a Transmission Control Protocol (TCP) connection; together they form the local and remote socket-address pair.
  • 5-tuple: The transport protocol plus the source and destination IP addresses and port numbers. These five values identify a single network flow, and firewall rules and monitoring tools commonly match on them.

A

Section titled “A”

B

Section titled “B”
  • Backend: The part of a system that runs behind the scenes, processing requests, applying business logic, and communicating with databases or other services. In proxy configurations, a backend may also be called an upstream.
  • Binary: A way of writing numbers using only 0 and 1. Devices store and process addresses in binary, and the decimal form of an IPv4 address is a convenience for people reading it.
  • Bit: A single 0 or 1, the smallest unit of data. An IPv4 address is 32 bits long.
  • Border Gateway Protocol (BGP): The routing protocol used to exchange reachability information between large networks, including networks that form the internet.
  • Broadcast: Traffic sent to every device in the same broadcast domain.
  • Broadcast address: The final address in a traditional IPv4 subnet, used to reach all IPv4 hosts on that subnet.
  • Broadcast domain: The group of devices that receive one another’s local broadcast traffic. Routers separate broadcast domains.

C

Section titled “C”

D

Section titled “D”
  • Decimal: The everyday base-10 way of writing numbers, using the digits 0 through 9. Each octet of an IPv4 address is shown in decimal so it is easier to read than binary.
  • Default-deny: A filtering policy that blocks traffic unless a rule explicitly permits it.
  • Default gateway: The router a host sends traffic to when the destination is outside the local subnet.
  • Default route: The route used when no more specific route matches a destination.
  • Dynamic Host Configuration Protocol (DHCP): A protocol that automatically supplies clients with IP addressing and other network settings.
  • DHCP lease: A time-limited assignment of network settings from a DHCP server to a client.
  • DHCP relay: A device or service that forwards DHCP messages between clients and a DHCP server on another network.
  • Discover, Offer, Request, Acknowledgment (DORA): The four-message exchange commonly used when a client obtains a new DHCP lease for IPv4.
  • Domain name: A name in the DNS hierarchy, such as example.com.
  • Domain Name System (DNS): The distributed system that stores information about names, including the IP addresses associated with them.

E

Section titled “E”
  • Encapsulation: The process of adding protocol information around data as it moves down the networking layers.
  • Ephemeral port: A temporary source port normally selected by the operating system for a client connection.
  • Ethernet: A common technology for moving frames across a local wired network.
  • Egress filtering: Filtering network traffic as it leaves a system or network boundary; egress traffic is also called outbound traffic.

F

Section titled “F”
  • Firewall: A control that permits or blocks network traffic according to rules.
  • Forward proxy: A proxy that makes requests on behalf of clients.
  • Frame: The unit of data carried across one local link, such as an Ethernet network.

H

Section titled “H”
  • Health check: A check that monitors whether a backend server is available so a load balancer can avoid sending traffic to an unhealthy server.
  • Host: A device with an IP address that can send or receive network traffic.
  • Hostname: A name assigned to a host. A hostname may be used as part of a complete domain name.
  • Hypertext Transfer Protocol (HTTP): An application protocol that uses requests and responses to transfer web content and other data.
  • Hypertext Transfer Protocol Secure (HTTPS): HTTP protected by Transport Layer Security (TLS).

I

Section titled “I”

L

Section titled “L”
  • Layer: A group of related networking responsibilities. Layer numbers usually refer to the Open Systems Interconnection model.
  • Lease: See DHCP lease.
  • Listening socket: A socket waiting for new traffic on a local address and port.
  • Load balancer: A hardware or software system that distributes connections, requests, or workloads among multiple backend servers.
  • Loopback: A special address used to reach the same host without sending traffic onto a physical network. IPv4 commonly uses 127.0.0.1.

M

Section titled “M”

N

Section titled “N”
  • Network address: The first address in a traditional IPv4 subnet, used to identify the subnet itself.
  • Network address translation (NAT): A gateway function that changes IP addressing information as traffic crosses a network boundary.
  • Next hop: The next router or local destination to which a host or router sends a packet.

O

Section titled “O”
  • Octet: A group of eight bits. An IPv4 address is written as four octets separated by dots, and each octet holds a value from 0 to 255.
  • On-link: Directly reachable on the local network without sending the packet through a router.
  • Open Systems Interconnection (OSI) model: A conceptual seven-layer reference model commonly used to describe where networking functions and problems occur.

P

Section titled “P”
  • Packet: The unit of data carried by IP from a source host toward a destination host.
  • Port: A number used by TCP or User Datagram Protocol (UDP) to direct traffic to the correct application or service.
  • Port Address Translation (PAT): A form of NAT that translates port numbers as well as addresses, allowing multiple private hosts to share one public IPv4 address.
  • Port forwarding: A rule that sends traffic arriving at one address and port to a chosen internal address and port.
  • Prefix length: The number after / in CIDR notation, showing how many leading address bits identify the network.
  • Private IPv4 address: An address from a range reserved for private networks and not routed across the public internet.
  • Proxy: A system that accepts one connection and creates another connection on behalf of a client or server.
  • Public IP address: An IP address that may be globally routed, subject to allocation and network policy.

R

Section titled “R”
  • Recursive resolver: A DNS server that accepts a client’s query, finds the answer, and commonly caches it.
  • Request for Comments (RFC): A publication in the RFC series that documents internet standards, protocols, practices, or other technical information.
  • Resource record: One item of DNS data, such as an address, mail-server, or alias record.
  • Reverse proxy: A proxy that accepts connections on behalf of one or more servers.
  • Route: An entry describing how traffic should reach a destination network or host.
  • Router: A device or system that forwards IP packets between networks.
  • Routing table: The list of routes a host or router uses to select a path.

S

Section titled “S”
  • Segment: Depending on context, this can mean a TCP unit of data or a portion of a local network. The linked section covers the TCP meaning.
  • Server: A device or application that listens for and responds to client requests.
  • Server Name Indication (SNI): Information sent during a TLS connection so a server can select the certificate for the requested hostname.
  • Session persistence: A load-balancing behavior that repeatedly sends one client’s traffic to the same backend. It is also called a sticky session.
  • Socket: A communication endpoint an application uses to send or receive network data.
  • Stateful firewall: A firewall that remembers active network flows and can recognize their return traffic.
  • Stateless firewall (stateless filter): A firewall that evaluates each packet independently using packet data and predefined rules, without retaining connection state.
  • Subnet: A range of IP addresses that share a network prefix.
  • Subnet mask: A 32-bit IPv4 mask that identifies the network-prefix bits and the remaining host bits of an address.
  • Switch: A device that forwards frames between devices on a local network using MAC addresses.

T

Section titled “T”
  • Transmission Control Protocol (TCP): A connection-oriented transport protocol that provides ordered, reliable byte delivery.
  • TCP/IP: The family of protocols used by the internet, named for the Transmission Control Protocol and Internet Protocol.
  • Transport Layer Security (TLS): A protocol that protects network traffic with encryption, integrity checking, and authentication.
  • Time to live (TTL): In an IP packet, a value reduced by each router to prevent routing loops. In a DNS record, the same abbreviation means how long a cache may reuse that record.

U

Section titled “U”

V

Section titled “V”

Z

Section titled “Z”
  • Zone: An administratively managed portion of the DNS namespace containing authoritative records.
Section titled “Source and Copyright Note”

This page links to source material; it does not reproduce source text, tables, or images. External material remains subject to its own copyright and license terms and is not included under this guide’s MIT License.

The RFC Editor publishes an information page for each RFC, which is the stable landing page this glossary links to. IANA’s protocol registries use a Creative Commons Zero (CC0) public-domain dedication. NIST describes most information on its sites as public information. Microsoft permits plain-text links to its content. Wikipedia text is available under the Creative Commons Attribution-ShareAlike 4.0 International license.