Cartoon Glossary

Little Dictionary of Network Terms

All 289 terms used on this site, each with its full name, a one-line explanation and a cartoon analogy. This list is the reference for how terms are written across the site.

Basic Concepts

Network: Two or more connected devices that can exchange data.AnalogyA group of classmates passing notes
Internet: The "network of networks" that links countless networks around the world.AnalogyThe world's road system
Node: Any device on a network that sends or receives data.AnalogyA station on a map
Link: The path connecting two nodes, either a network cable or a wireless signal.AnalogyThe tracks between stations
LANLocal Area Network: A small-area network, such as a home or a school.AnalogyA lane inside a housing estate
WANWide Area Network: A large network that spans cities or countries.AnalogyMotorways between cities
Server: A computer dedicated to providing services (such as web pages, video or email) to other devices.
ISPInternet Service Provider: A company that provides internet access and connects your network to the Internet.
Ethernet: The most common wired LAN technology, which connects devices with network cables.
SSIDService Set Identifier: The name of a Wi-Fi network, which you see in your phone's Wi-Fi settings.AnalogyThe sign at the front door

Addresses

MAC AddressMedia Access Control Address: A 48-bit identifier given to a network interface at the factory, such as 00:00:5E:00:53:01.AnalogyA name tag sewn onto your clothes
Hexadecimal: A way of writing numbers using 16 symbols: 0–9 and A–F.
OUIOrganizationally Unique Identifier: The first 3 bytes of a MAC Address, which identify the manufacturer.AnalogyThe school badge on the name tag
Broadcast Address: A special address for sending to every device on the same network (for MAC: FF:FF:FF:FF:FF:FF).AnalogyA school announcement
Media Access Control: What "MAC" stands for in MAC Address. It has nothing to do with "Mac" computers.
Bit: The smallest unit of computer data, which can only be 0 or 1.
Byte: 8 Bits make 1 Byte.
Network Interface: The part a device uses to join a network, such as a Wi-Fi chip or a cable socket. Each one has its own MAC Address.AnalogyThe spot where the name tag is pinned
NICNetwork Interface Card: A Network Interface installed in a computer as a card.
IEEEInstitute of Electrical and Electronics Engineers: The international organisation that sets technical standards such as Ethernet and Wi-Fi, and assigns OUIs to manufacturers.
Unicast: Sending to just one device.AnalogyA letter with the recipient's name on it
Broadcast: Sending to every device on the same LAN at once.AnalogyA school announcement
Random MAC Address: A random address the system uses instead of the real MAC Address, to protect privacy.AnalogyA temporary name tag
IP AddressInternet Protocol Address: A device's logical address on a network, used for finding the way.AnalogyYour home's mailing address
IPv4Internet Protocol version 4: A 32-bit IP Address written in dotted decimal, such as 192.168.1.10.
IPv6Internet Protocol version 6: A 128-bit IP Address written as groups of hexadecimal, such as 2001:db8::1.
Public IP: An IP Address that can be reached directly on the Internet.AnalogyA building's street address
Private IP: An IP Address used only inside a private network.AnalogyA flat number inside the building
Subnet: A smaller section made by splitting a network.AnalogyThe range of house numbers on one street
Subnet Mask: Shows which bits of an IP Address belong to the network part, such as 255.255.255.0 (that is, /24).AnalogyA ruler that marks out the street boundaries
Default Gateway: The Router that a device hands data to when it is meant for another network.AnalogyThe front gate to the outside world

Packets and Layers

Packet: A small piece that data is cut into for sending across a network.AnalogyOur hero Pip
Header: The front part of a packet that records details such as the sender and the recipient.AnalogyPip's backpack
Payload: The actual data a packet is delivering.AnalogyThe letter inside the backpack
Frame: The wrapping used when data travels inside a LAN; its header holds the recipient's and sender's MAC Addresses.AnalogyA delivery label for inside the LAN
Encapsulation: The process in which each layer wraps its own header around the data.AnalogyGift wrap, layer after layer
TCP/IP Model: The four-layer network model used for teaching on this site.
OSI Model: A seven-layer reference model, used on this site for reference only.
Network Layers: Splitting the work of network communication into layers, each with just one job.AnalogyGift wrap, layer after layer
Application Layer: The top layer of the TCP/IP Model, which prepares the content to send, such as HTTPS and DNS.AnalogyThe contents of the letter
Transport Layer: Decides which program on the other side gets the data and whether delivery is guaranteed, such as TCP and UDP.AnalogyThe envelope
Internet Layer: Decides which network and which device the data goes to, by writing in the IP Address.AnalogyPip's backpack
Link Layer: Gets the data to the next stop on this stretch of the LAN, by writing in the MAC Address; for example Ethernet and Wi-Fi.AnalogyThe delivery bag
Decapsulation: The process in which each layer removes its own label as data is received.AnalogyUnwrapping the gift
Segment: The data wrapping used by the Transport Layer when it uses TCP.
Datagram: The data wrapping used by the Transport Layer when it uses UDP.
MTUMaximum Transmission Unit: The largest amount of data that can be sent at one time; on Ethernet it is commonly 1,500 Bytes.AnalogyHow much the backpack can hold

Network Devices

Hub: An old-style device that repeats whatever it receives out of every port.AnalogySomeone shouting across a classroom
Switch: Sends data to the right device inside a LAN, based on the MAC Address.AnalogyOcto
Router: Forwards packets between different networks, based on the IP Address.AnalogyRue (the post office)
Access Point: A device that lets other devices join a wired network over Wi-Fi.
MAC Address Table: A table in a Switch that records which MAC Address is on which Port.AnalogyOcto's notebook
Routing Table: The table a Router uses to decide where to send a packet.AnalogyRue's map
ARPAddress Resolution Protocol: A way to find the MAC Address that matches an IP Address.AnalogyShouting "Who is so-and-so?"
ARP Cache: A table in which a device remembers which IP Address matches which MAC Address; you can view it with arp -a.
Collision: Two devices send at the same time on a shared line and their signals crash into each other.AnalogyTwo people talking at once
Default Route: The 0.0.0.0/0 entry in the Routing Table, used when no more specific route is found.Analogy"Everywhere else, take this road"
Neighbor Discovery: The method IPv6 uses in place of ARP.
Firewall: A device or piece of software that decides, by rules, whether a packet may pass.AnalogyPango

Services and Protocols

Protocol: A set of rules that devices follow when they communicate.AnalogyEveryone speaking the same language
TCP/IPTransmission Control Protocol / Internet Protocol: The whole set of communication rules shared by all devices on the Internet, named after TCP and IP.AnalogyThe language the whole world shares
Email: A message sent over a network; the address uses "@" to separate the user name from the domain.
Web Page: One page of content on the World Wide Web, which can contain text, pictures and hyperlinks.
Hyperlink: A link that jumps to another web page or location when you click it.AnalogyA magic door
Browser: An application used to open and read web pages.
DNSDomain Name System: A system that turns web addresses into IP Addresses.AnalogyThe Internet's phone book (Dr. Lookup)
Domain Name: An easy-to-remember name for a website, such as example.com.AnalogyA name in the phone book
Resolver: A server that looks up answers from other DNS servers on your behalf.AnalogyThe librarian who checks the phone book for you
Root Server: The top-level starting point of a DNS lookup.AnalogyThe master directory
TLDTop-Level Domain: The last part of a web address, such as .com or .hk.AnalogyA volume of the phone book
Authoritative Server: The DNS server that holds the official answer for a domain.AnalogyThe person answering the phone themselves
Cache: Remembering an answer for a while so that next time is faster.AnalogyA frequently used number stuck on the fridge
DNS Record: One entry in the DNS phone book.AnalogyOne line in the phone book
A RecordAddress Record: A DNS record that maps a name to an IPv4 address.
AAAA Record: A DNS record that maps a name to an IPv6 address.
CNAME RecordCanonical Name Record: A DNS record that points one name to another name (an alias).AnalogyA nickname
MX RecordMail Exchange Record: A DNS record that says which server should receive a domain's email.
DHCPDynamic Host Configuration Protocol: A protocol that automatically gives devices an IP Address and other settings.AnalogyThe hotel front desk handing out room cards
NATNetwork Address Translation: Lets many devices that use Private IPs share one Public IP.AnalogyThe building's management office receiving mail for everyone
Lease Time: How long DHCP lends an IP Address to a device.AnalogyHow long a hotel room card is valid
DORADiscover, Offer, Request, Acknowledge: The four steps of DHCP: Discover, Offer, Request, Acknowledge.
PATPort Address Translation: A kind of NAT that rewrites the Port number as well; home Routers use this kind.
Port Forwarding: Setting up a Router in advance so that packets sent to a certain Port are handed to a particular device.AnalogyThe management office's forwarding instructions
TCPTransmission Control Protocol: A transport protocol that delivers data reliably and in order.AnalogyRegistered mail
UDPUser Datagram Protocol: A fast transport protocol that does not guarantee delivery.AnalogyA postcard
Port: In TCP/UDP, a number (0 to 65535) that tells apart different services on the same device, such as 443. On devices like a Switch, it is the socket where you plug in a network cable.AnalogyA mailbox number in the building
Three-way Handshake: The three steps TCP takes before it sets up a connection.Analogy"Hello?" "Hello!" "Let's begin!"
SYNSynchronize: The first step of the TCP three-way handshake, meaning "I would like to start a connection".Analogy"Hello?"
ACKAcknowledgement: A message from the recipient saying "I've received everything up to here".AnalogyA signed receipt
Sequence Number: A number TCP gives to data, used for confirming, resending and putting things in order.AnalogyThe number on Pip's backpack
FINFinish: The message TCP sends when it ends a connection.Analogy"Goodbye!"
QUIC: A newer transport technology built on UDP that handles resending and encryption itself; HTTP/3 uses it.
URLUniform Resource Locator: The full address of a resource on the web, such as https://example.com/.
HTTPSHyperText Transfer Protocol Secure: A protocol that sends web pages in encrypted form.AnalogyA sealed envelope
HTTPHyperText Transfer Protocol: The protocol a browser uses to ask a server for web pages; the default Port is 80.AnalogyA postcard
TLSTransport Layer Security: The technology HTTPS uses to encrypt data and prove the server's identity.AnalogyThe seal and wax stamp
Digital Certificate: An electronic document that proves a server really belongs to a certain domain.AnalogyThe server's ID card
HTMLHyperText Markup Language: The language used to write web pages.
Status Code: A three-digit number attached to a server's reply, such as 200 or 404.
Scheme: The start of a URL that says which method to use to fetch the data, such as https.
Wi-Fi: A technology that connects to a LAN using radio waves.
WPAWi-Fi Protected Access: Technology that encrypts Wi-Fi signals, such as WPA2 and WPA3.

Network Mini-Stories

Redundancy: Having a spare line or a spare device, so that if one fails the other can take over.AnalogyAn extra backup road
Switching Loop: Links between Switches that form a circle, so data goes round and round inside it.AnalogyGoing round a roundabout and never finding the exit
Broadcast Storm: A broadcast that keeps getting copied and forwarded around a loop until it clogs the whole network.AnalogyA school announcement that keeps replaying until nobody can hear anything
STPSpanning Tree Protocol: A protocol that lets Switches automatically find and temporarily shut down extra links to avoid loops.AnalogyOcto temporarily closing off the extra roads
Flooding: When a Switch does not know the recipient, or receives a Broadcast, it sends the Frame out of every Port except the one it came from.AnalogyOcto delivering a letter to every doorway
Forwarding: When a Switch knows the recipient, it sends the Frame out of that one Port only.AnalogyOcto delivering straight to the door
Filtering: If the recipient and sender are on the same Port, the Switch does not send the Frame out again and simply drops it.
Aging Time: How long an entry in the MAC Address Table is kept after it was last used before it is deleted (many Switches default to 300 seconds).AnalogyA notebook that gets tidied up regularly
Layer 2: The industry's name, borrowed from the OSI Model, for technology that looks at MAC Addresses and works inside one LAN, such as a Switch.AnalogyOcto's world
Layer 3: The industry's name, borrowed from the OSI Model, for technology that looks at IP Addresses and works between different networks, such as a Router.AnalogyRue's world
Single Point of Failure: A weakness where one thing going wrong brings the whole system to a halt.AnalogyA single-plank bridge
TTLTime to Live: A number in the IP packet header that drops by 1 at every Router and gets the packet discarded when it reaches 0; DNS answers also have a TTL, which says how many seconds they may be cached.AnalogyA packet's lifespan
BPDUBridge Protocol Data Unit: The messages Switches exchange with each other in STP.AnalogyLittle letters between Switches
Bridge ID: Made of a priority value and a MAC Address, and used to choose the Root Bridge.
Root Bridge: The Switch chosen by STP to be the root of the whole tree.AnalogyThe tree root
Root Port: On a Switch that is not the Root Bridge, the Port with the lowest cost to reach the Root Bridge.AnalogyThe shortest way home
Designated Port: The Port on each link that is responsible for forwarding Frames.
Path Cost: A value STP uses to compare how far routes are; the faster the cable, the lower the cost (for example, 1 Gbps is 4).
RSTPRapid Spanning Tree Protocol: A newer version of STP that usually recovers within a few seconds.
Layer 1: The layer that handles only electrical signals or radio waves and does not understand the contents of a Frame, such as a Hub.
CSMA/CDCarrier Sense Multiple Access with Collision Detection: The rule on a shared line: "listen before talking, stop if there is a crash, then wait a random time and try again".AnalogyListening first in a meeting, and stopping if you talk over someone
Jam Signal: A signal sent after a Collision is detected, telling other devices that a crash has happened.AnalogyShouting "We crashed!"
Backoff: After a Collision, each device waits a random time before sending again.AnalogyEach person picks a random number before speaking again
Half-duplex: Can either send or receive, but not both at the same time.AnalogyA walkie-talkie
Full-duplex: Can send and receive at the same time.AnalogyA telephone
Collision Domain: A group of devices that will crash into each other if they send at the same time.AnalogyThe same classroom
Broadcast Domain: The area a Broadcast can reach; each Router Port and each VLAN is its own separate Broadcast Domain.AnalogyThe area a school announcement can be heard
ARP Request: An ARP message that uses a Broadcast to ask "Who has this IP Address?".AnalogyShouting across a classroom to find someone
ARP Reply: An ARP message that uses Unicast to answer "That's me, and here is my MAC Address".AnalogyRaising a hand to answer
ARP Spoofing: Pretending to be someone else when answering ARP, so that data is sent to the attacker by mistake.AnalogyRaising your hand while pretending to be someone else
Beacon: A wireless message that an Access Point sends out regularly to announce details such as its SSID.AnalogyThe sign light at the front door
BSSIDBasic Service Set Identifier: The MAC Address on the wireless side of an Access Point.
Channel: A smaller slice of a Wi-Fi frequency band; devices on the same channel share the air.AnalogyA radio channel
CSMA/CACarrier Sense Multiple Access with Collision Avoidance: The rule Wi-Fi uses: listen first, wait a random time before sending, and confirm with an ACK.AnalogyRaising a hand and waiting a moment before speaking
Authentication: The step of confirming who the other side is.
Association: The step in which a wireless device registers to join an Access Point.AnalogyChecking in
VLANVirtual LAN: Groups the Ports of one Switch so that each group becomes its own separate LAN (Broadcast Domain).AnalogyA screen divider in a room
VLAN ID: A 12-bit number that identifies a VLAN, from 1 to 4094, written in the 802.1Q tag.AnalogyA zone number
Access Port: A Switch Port that connects ordinary devices and belongs to just one VLAN.
Inter-VLAN Routing: Forwarding data between different VLANs through a Router or a Layer 3 Switch.AnalogyA bridge between two zones
Layer 3 Switch: A Switch that can also forward Packets by IP Address, combining the functions of a Switch and a Router.AnalogyOcto, who has learned to read IP Addresses
Trunk: A link that can carry several VLANs at once, using 802.1Q tags to tell which VLAN each Frame belongs to.AnalogyA private bus carrying students from different schools
802.1Q: The VLAN tagging standard set by the IEEE: a 4-Byte tag inserted into a Frame states the VLAN ID.AnalogyThe school badge on a student's chest
Native VLAN: The VLAN that is sent without a tag on a Trunk; the default is VLAN 1, and both ends must be set the same.
Allowed VLAN: The list of VLANs that are permitted to pass over a Trunk.
Bridge: The ancestor of the Switch; STP terms still use this name.
Blocked Port: A Port in STP that has paused forwarding Frames and only listens for BPDUs; it can be brought back into use if a link fails.AnalogyA "closed to traffic" sign
Bridge Priority: The first half of the Bridge ID; the default is 32768, it can be changed in multiples of 4096, and the lower value wins.
Link Aggregation: Bundling several network cables into one logical link that sends together; if one breaks, the rest keep working. Also known as LAG, Port-channel, EtherChannel, Bonding or Teaming.AnalogyWidening a one-lane bridge into a multi-lane road
LACPLink Aggregation Control Protocol: A protocol that two devices use to confirm and monitor which cables belong to the same Link Aggregation group.
LACPDULink Aggregation Control Protocol Data Unit: The messages that LACP exchanges.
Hash: A way of putting data into a formula to get a fixed number; Link Aggregation uses it to decide which cable each conversation takes, and a Load Balancer uses it so that the same user usually goes to the same server.AnalogyThe lane signs at the start of the bridge
Bandwidth: The most data a link can carry per second, such as 1 Gbps.AnalogyHow many lanes a road has
Next Hop: The next stop a Router hands a Packet to (the IP Address of the next Router).AnalogyThe next place to change buses
Router-on-a-stick: A Router connects to a Switch with just one Trunk and sets up a Subinterface for each VLAN.
Subinterface: A virtual interface created by configuration on one physical Port, each belonging to a VLAN.AnalogyOne Port, two identities
SVISwitch Virtual Interface: A virtual interface on a Layer 3 Switch that stands for a VLAN, and can be given an IP Address to act as the Default Gateway.
ACLAccess Control List: A list of rules checked from top to bottom to decide whether a packet is allowed through or blocked.AnalogyPango's list
Subnetting: Splitting a set of IP Addresses into several smaller Subnets.AnalogyCutting a cake
Network Address: The first address of a Subnet (with the device part all 0s), which stands for the whole network and cannot be given to a device.
Prefix Length: The number after the /, which says how many bits at the front of an IP Address belong to the network part.
CIDRClassless Inter-Domain Routing: A way of writing a network range as "address/prefix length" (for example /26).
Static Route: A Route that a network administrator adds to the Routing Table by hand.AnalogyA road drawn onto the map by hand
Connected Route: A network directly attached to a Router's own Port, which appears in the Routing Table automatically.
Longest Prefix Match: When several Routes match, choosing the one with the longest prefix, which is the most specific.AnalogyThe more detailed the address, the higher the priority
Dynamic Routing: Routers automatically exchange route information and update their Routing Tables.AnalogyThe Rues swapping news
OSPFOpen Shortest Path First: The most widely used Link State dynamic routing protocol.
RIPRouting Information Protocol: An early Distance Vector dynamic routing protocol that measures distance by how many Routers a packet passes through.
Distance Vector: A dynamic routing method that only listens to neighbours saying "this is how far it is to get there".AnalogyAsking only your neighbours for directions
Link State: A dynamic routing method in which every Router holds the full map and works out routes itself.AnalogyEveryone holding a full map
Hello: A greeting message an OSPF Router sends to its neighbours regularly, to get to know them and confirm they are still online.Analogy"I'm here!"
Dead Interval: How long without a Hello before a neighbour is considered lost (commonly 40 seconds by default).
LSALink State Advertisement: A Router's report on which networks and neighbours it is connected to and what the Costs are.AnalogyA news letter
LSDBLink State Database: The complete map put together from all the LSAs.AnalogyThe same map for everyone
SPFShortest Path First: The algorithm that works out the route with the lowest total Cost, also known as Dijkstra's algorithm.
Cost: The "distance" value OSPF sets for each link; usually the faster the link, the lower the value.AnalogyHow long the journey is
Convergence: The process in which all Routers work out consistent, correct routes again after the network changes.
Area: OSPF groups Routers, and each group shares one LSDB, so the map doesn't get too big.
BGPBorder Gateway Protocol: The protocol that different ISPs and organisations on the Internet use to exchange routes.
NAT Table: A table in which a Router records which internal address and Port matches which external Port.AnalogyThe management office's record book
Ephemeral Port: A temporary Port number a device picks when it starts a connection, such as 50000.AnalogyA temporary mailbox number
CGNATCarrier-Grade NAT: An ISP doing NAT once more inside its own network, so that many homes share one Public IP.AnalogyAnother management office outside the building's own one
VPNVirtual Private Network: Connects back to another network through an encrypted tunnel, as if you were inside it.AnalogyA secret tunnel
IPInternet Protocol: The rules for sending packets stop by stop to their destination by IP Address; it only does its best and does not guarantee delivery.
Window: How much data TCP may send without yet being acknowledged; the recipient also uses it to tell the other side how much space it has left.
Buffer: A memory area that holds data temporarily, for example data that arrived out of order.AnalogyA waiting area off to one side
Duplicate ACKDuplicate Acknowledgement: The recipient sends the same ACK number again, meaning some data in between has not arrived.Analogy"I'm still waiting for 2001!"
Retransmission: The sender sends data again that it suspects was lost.AnalogyPosting another copy
Fast Retransmit: Resending immediately after receiving 3 Duplicate ACKs, without waiting for the timer to run out.
Flow Control: The recipient uses the Window to tell the sender how much space it has left, so that it doesn't get overwhelmed.
Congestion Control: The sender estimates whether the network is jammed and adjusts its sending speed automatically.
ICMPInternet Control Message Protocol: Messages that Routers and computers use to report errors and run tests.AnalogyThe network's report letters
Ping: A tool that sends an ICMP Echo Request and waits for an Echo Reply, to test whether a device can be reached and how long the round trip takes.AnalogyThe network doctor's stethoscope
Traceroute: A tool that uses a TTL that grows step by step to list every stop a packet passes along the way; the Windows command is tracert.AnalogyCalling the roll stop by stop
Echo Request: The ICMP message that Ping sends out (Type 8).Analogy"Are you there?"
Echo Reply: The ICMP message that answers an Echo Request (Type 0).Analogy"I'm here!"
Time Exceeded: The ICMP message (Type 11) a Router sends back when TTL drops to 0 and the packet is discarded.
Destination Unreachable: The ICMP message (Type 3) sent back when a packet cannot be delivered to its destination.Analogy"Road closed"
Round-trip Time (RTT): The time from sending data to receiving the reply.
Hop: Each stop (each Router) that a packet passes on its way.AnalogyOne stop
Encryption: Using maths to turn data into scrambled text that only someone holding the key can understand.AnalogySealing the envelope
CACertificate Authority: A trusted organisation that checks who owns a domain and signs Digital Certificates.AnalogyThe immigration office that issues ID cards
Private Key: A key kept only by its owner, used for signing or decrypting, and paired with the public key on the certificate.AnalogyA stamp that only its owner has
Diffie-Hellman Key Exchange: A method where both sides swap only "key ingredients" and each works out the same key; the key itself is never sent over the network.AnalogyEach mixing the same colour on their own
Symmetric Encryption: Encrypting and decrypting with the same key; it is fast, and TLS uses it to protect data after the handshake.
SNIServer Name Indication: The domain name you want to connect to, written in plain text inside the ClientHello, so the server can bring out the right certificate.AnalogyThe company name written on the outside of the envelope
ECHEncrypted Client Hello: A newer technology that also encrypts details in the ClientHello, such as the website name.
Tunnel: A channel that encrypts a whole packet and wraps it inside another packet for sending.AnalogyPip sitting inside a shipping crate
VPN Client: The software on a device that sets up the tunnel, encrypts and encapsulates the data.
VPN Server: The device or software at the other end of the tunnel that verifies identity and unpacks and decrypts the data.AnalogyRue unpacking the crate back home
Full Tunnel: A setup where all data is sent through the VPN tunnel.
Split Tunnel: A setup where only data going to specified networks is sent through the VPN tunnel.
Remote-access VPN: A VPN that lets one device connect back to a network from outside.
Site-to-site VPN: A VPN that keeps two networks permanently connected through their Routers.
WireGuard: A VPN software that commonly uses UDP Port 51820 by default.
IPsecInternet Protocol Security: A set of encryption protocols often used for Site-to-site VPNs.
OpenVPN: A common VPN software.
First Match: An ACL follows the first rule that matches and does not look at the rules below it.
Implicit Deny: The invisible "block everything" rule at the end of an ACL.AnalogyYou're not on the list. No entry!
State Table: A table in which a Stateful Firewall records connections started from inside, so that replies can be let in.
Stateful Firewall: A Firewall that remembers connections and automatically lets replies through.
Stateless Firewall: A Firewall that checks each packet on its own and doesn't remember earlier connections.
Next-Generation Firewall: A Firewall that recognises applications as well as addresses and Ports.
Least Privilege: A security principle: give only the permissions that are truly needed, and nothing more.AnalogyOpening only the doors that are really needed
Telnet: An old, unencrypted remote login service that uses TCP Port 23.
Load Balancer: A device or software in front of a group of servers that hands each request to one of them.AnalogyThe host at a restaurant door showing guests to tables
High Availability: A design that lets a system keep working even when some devices fail.AnalogyA backup colleague for every post
VIPVirtual IP Address: An outward-facing IP Address that doesn't belong permanently to one machine and can be taken over by another.
Round Robin: A distribution method that hands requests to each server in turn, in order.
Least Connections: A distribution method that hands the request to the server with the fewest connections right now.
Health Check: A Load Balancer regularly checking whether a server is responding normally.Analogy"Are you OK?"
Active-Standby: One device does the work while another waits on standby to take over.
Active-Active: Two devices share the work at the same time.
Heartbeat: A message two devices send each other regularly to confirm the other is still running.Analogy"I'm still here!"
Failover: The process in which a backup device automatically takes over when the main device fails.
Data Center: A building that houses large numbers of servers and network devices.
Learning: The action of a Switch noting the sender's MAC Address and the source Port in the MAC Address Table; in STP it is also the stage before a Port starts forwarding, when it "only learns and does not forward".AnalogyOcto writing in the notebook
Listening: The STP stage in which a Port only sends and receives BPDUs, and has neither learned nor started forwarding.
Blocked: The STP Port state in which forwarding of Frames is paused (also called Blocking).AnalogyA "closed to traffic" sign
Tagged: A Frame sent carrying an 802.1Q VLAN tag, used on Trunks.
Untagged: A Frame sent without a VLAN tag, used on Access Ports and the Native VLAN.
Neighbor: A Router directly connected to another and exchanging Hellos with it, such as an OSPF neighbour.

History

Telegraph: A way of communicating in the 19th century that sent text as electrical signals.
Morse Code: A code that uses long and short signals (dots and dashes) to stand for letters and numbers.
Telephone: A way of communicating by sending voice over wires, which appeared in 1876.
Circuit Switching: A method that reserves a whole dedicated line for a pair of users during a call, such as the traditional telephone.
Packet Switching: A method that cuts data into packets and sends each one off to find its own way, with the lines shared by everyone.
ARPANETAdvanced Research Projects Agency Network: An early packet-switching network that started up in 1969, the forerunner of the Internet.
World Wide Web: A service of web pages and hyperlinks developed in 1989–91.
Mobile: Going online anytime, anywhere, with a portable device such as a phone.
Cloud: Using services provided by remote servers over the Internet.

One-to-Many Delivery

Multicast: Sending one copy of data to a group, which only devices that have joined the group will accept.AnalogyA notice for an interest class
Multicast Group: A set of recipients represented by one Multicast address, such as 233.252.0.1.AnalogyAn interest class
Anycast: Several servers share the same address, and data goes to the nearest one.AnalogyCalling a chain store's main number and being answered by the nearest branch
IPTVInternet Protocol Television: Television shown over an IP network; live broadcasts usually use Multicast.
mDNSMulticast DNS: A way for devices on a LAN, such as printers and speakers, to find each other using Multicast.
PIMProtocol Independent Multicast: A protocol that lets Routers carry Multicast across networks to wherever it is needed.
IGMPInternet Group Management Protocol: A protocol that devices use to tell a Router that they are joining or leaving a Multicast group.AnalogyHanding in a sign-up form for an interest class
Membership Report: The IGMP message in which a device says "I would like to join this group", commonly called a Join.AnalogyA sign-up form
General Query: A message the IGMP Querier sends regularly to ask "Who still wants to watch?".AnalogyRoll call
Group-Specific Query: After a device leaves, the Router asks once more whether anyone is still in that group.
IGMP Querier: The device on a LAN, usually a Router, that sends out IGMP Queries regularly.AnalogyThe teacher who takes the roll call
IGMP Snooping: A Switch listens to IGMP messages and sends Multicast only to the Ports that have members.AnalogyA receptionist who glances at the sign-up forms
IGMP Proxy: A home Router signs up to Multicast groups with the ISP on behalf of the devices in the home.
Multicast Router Port: The Port on a Switch connected to a Multicast Router, to which all sign-up messages must be sent.
Fast Leave: Stops sending as soon as a Leave arrives, without waiting for the Router to query again.
MLDMulticast Listener Discovery: The IPv6 version of IGMP.

Network Buddies Pro

PVST+Per-VLAN Spanning Tree Plus: Cisco's version of STP, which works out a separate tree for each VLAN.AnalogyEach VLAN has its own park ranger
Rapid PVST+Rapid Per-VLAN Spanning Tree Plus: Cisco's version of RSTP, which works out a separate RSTP tree for each VLAN and usually recovers within a few seconds.
MSTMultiple Spanning Tree: The IEEE 802.1s standard, which groups several VLANs so that each group shares one tree.
Extended System ID: The 12-bit part of the Bridge ID that is filled in automatically with the VLAN number.
Port Role: The role STP gives each Port, such as Root, Designated or Alternate.
Port State: Whether a Port is currently learning MAC Addresses and forwarding Frames; RSTP has three states: Discarding, Learning and Forwarding.
Alternate Port: In RSTP, another backup route to the Root Bridge, which can step in at once if the Root Port fails.AnalogyA backup road that is already prepared
Backup Port: When a Switch has two Ports connected to the same network segment, one of them acts as the backup.
Discarding: The RSTP Port state in which a Port neither learns nor forwards ordinary Frames.
Point-to-Point: A full-duplex link connecting only two devices, on which RSTP can do a quick handshake.
Proposal / Agreement: The handshake in which two RSTP Switches confirm directly with each other so that a Port can start forwarding quickly.Analogy"May I come over?" "Yes!"
Edge Port: A Port that connects an end device, with no Switch on the other end.AnalogyThe last home at the end of the estate
PortFast: A Cisco setting that treats a Port as an Edge Port and forwards as soon as a cable is plugged in.
Topology Change: STP telling the whole network that "the tree has changed", so that Switches clear out part of their MAC Address Tables.
BPDU Guard: As soon as an Edge Port receives a BPDU, the Port is put into err-disabled.AnalogyThe door guard who locks up on seeing an unexpected visitor
Root Guard: Stops the Switch beyond a Port from taking over as Root Bridge, and recovers automatically once it stops.
Loop Guard: Stops a Root Port or Alternate Port from forwarding rashly if it suddenly stops receiving BPDUs.
BPDU Filter: Makes a Port stop sending and receiving BPDUs; using it in the wrong place can cause a loop.
err-disabled: The state in which a Switch shuts down a Port automatically because it detected an error; it must be recovered manually or automatically.
Port Priority: The first half of the Port ID; the default is 128, and it is used when breaking a tie.