The Codex Lab

Field notes / 007

Networking Glossary

Every term the labs use, defined in plain language, with a link to the explorer where you can see it working. Search or filter by topic — nothing leaves your browser.

137 terms

4

4-way handshakeWireless Auth
Four EAPOL-Key messages that turn the PMK into a pairwise key, deliver the group key, and confirm installation — without ever transmitting the key itself.Step through the handshake

8

802.11r fast transitionWireless Auth
Pre-negotiating key material with the next AP so a roam skips the full authentication — the difference between a seamless call and a dropped one.Compare roaming options
802.1XWireless Auth
A port-based access control framework. The client (supplicant) proves its identity to an authentication server through the AP or switch (authenticator) before any traffic is forwarded.802.1X Explorer

A

AdjacencyOSPF
A fully synchronized relationship between two OSPF neighbors. Only adjacent routers exchange link-state databases.Single area
Administrative distanceOSPF
A router's preference ranking between routing sources when more than one offers the same prefix. Lower distance is trusted first.Cost & SPF
Antenna gainRF & Wireless
How much an antenna concentrates energy in a chosen direction rather than adding power. Gain trades coverage width for reach.RF Math Explorer
AreaOSPF
A grouping of OSPF routers that share one link-state database. Areas limit flooding and shrink the shortest-path calculation.Multi-area design
Area Border RouterABROSPF
A router with interfaces in two or more areas. It summarizes prefixes between them and keeps a database per area.Multi-area design
AuthenticatorWireless Auth
The access point or switch in the middle. It relays EAP between client and server and never sees the credential itself, but it does receive the resulting key material.See the roles in the exchange
Autonomous systemASGeneral
A network under one routing administration, identified by an AS number. Interior protocols run inside; BGP runs between.OSPF ↔ BGP redistribution
Autonomous System Boundary RouterASBROSPF
A router that injects routes learned from another protocol — BGP, static, EIGRP — into OSPF as external routes.OSPF ↔ BGP redistribution

B

Backbone area (Area 0)OSPF
The transit area every other area must connect to. Inter-area traffic passes through Area 0 by design.Multi-area design
BeamformingMU-MIMO
Shaping a signal across multiple antennas so it adds up at the intended receiver and cancels elsewhere. Explicit beamforming needs feedback from the client.Step through sounding
Binary (base 2)Number Bases
Numbers built from only 0 and 1. Every masking, matching and addressing decision in networking happens at this level.Convert and see the bits
BitNumber Bases
A single binary digit — one 0 or one 1. The smallest unit of information.Number Bases
Bitwise ANDNumber Bases
The operation that combines an address with its mask to produce the network address: a bit survives only if both inputs are 1.Subnetting Lab
Blocked portSpanning Tree
A port intentionally left non-forwarding so the redundant link it belongs to cannot create a loop. It still listens, ready to take over.Find the dark cable
Border Gateway ProtocolBGPOSPF
The path-vector protocol that routes between autonomous systems on the internet. It chooses on policy attributes rather than a single cost metric.OSPF ↔ BGP redistribution
Bridge IDBIDSpanning Tree
The value switches compare during the root election: bridge priority first, then MAC address as the tiebreaker. Lowest wins.Spanning Tree Explorer
Bridge prioritySpanning Tree
A configurable number (default 32768, adjustable in steps of 4096) that lets you decide which switch becomes root instead of leaving it to MAC addresses.Drag a priority and watch the tree change
Bridge Protocol Data UnitBPDUSpanning Tree
The small frame switches exchange to advertise bridge IDs and path costs. BPDUs drive the election and detect topology changes.Spanning Tree Explorer
Broadcast addressAddressing & Subnetting
The last address in a subnet, with all host bits set to one. Traffic sent to it reaches every host on that subnet.Subnetting Lab
Broadcast domainGeneral
The set of devices a broadcast frame reaches — one VLAN or subnet. Routers separate broadcast domains.
Broadcast stormSpanning Tree
Broadcast frames circling a Layer 2 loop endlessly, multiplying until the network saturates. The problem spanning tree exists to prevent.Spanning Tree Explorer
Byte / octetNumber Bases
Eight bits, holding a value from 0 to 255. 'Octet' is the networking term used for each part of an IPv4 address.See octet breakdowns

C

Cable and connector lossRF & Wireless
Power lost between radio and antenna. It scales with cable length, cable type and frequency, and every connector adds a little more.Build a gain/loss chain
Captive portalWireless Auth
A web page that gates forwarding after the client has already joined. It authorizes traffic but does not encrypt the radio link.Step through a portal join
Channel soundingMU-MIMO
Measuring the radio channel to each client so the AP knows how to steer to it. The AP sends a sounding frame and the client reports what it heard.Watch the sounding cycle
Classless Inter-Domain RoutingAddressing & Subnetting
The addressing model that replaced fixed Class A/B/C boundaries with arbitrary prefix lengths.Subnetting Lab
Client groupingMU-MIMO
Choosing which clients can share one MU-MIMO transmission. Clients whose channels look too alike are split into different groups instead.Try good and bad groupings
Compressed beamforming reportMU-MIMO
The client's reply to a sounding frame, describing the channel it measured in compressed form. Clients that cannot send one can never be grouped.Collect channel feedback
ConvergenceSpanning Tree
The period after a change during which every device recalculates and settles on a consistent view of the topology. Traffic may be disrupted until it completes.Fail a link and watch it settle

D

dBiRF & Wireless
Antenna gain compared with a theoretical isotropic radiator. Higher dBi means a narrower, more focused pattern.RF Math Explorer
dBmRF & Wireless
Absolute power referenced to one milliwatt. 0 dBm is 1 mW, 20 dBm is 100 mW, and every 3 dB roughly doubles or halves power.Convert dBm to milliwatts
DecibeldBRF & Wireless
A logarithmic ratio between two power levels. Gains and losses in dB simply add and subtract, which is why RF math uses them.RF Math Explorer
Decimal (base 10)Number Bases
Everyday base-10 numbers. Used for the human-readable octets in dotted-decimal IPv4 addresses.Number Bases
Default gatewayAddressing & Subnetting
The router address a host sends traffic to when the destination is outside its own subnet.
Designated portSpanning Tree
The forwarding port on each network segment — the port closest to the root on that segment. Every segment has exactly one.See port roles on the topology
Designated RouterDROSPF
On a multi-access segment, the router elected to represent the segment and reduce the number of adjacencies needed. The BDR is its standby.Single area

E

EAP-TEAPWireless Auth
Tunnel EAP with chained inner methods, so the machine and the user are both authenticated in one session instead of through separate workarounds.Compare EAP types
EAP-TLSWireless Auth
Mutual certificate authentication. Nothing phishable is transmitted, at the cost of enrolling a certificate on every client.Run an EAP-TLS exchange
EAP-TTLSWireless Auth
Similar to PEAP but with a flexible inner method, which lets it hand even a plaintext password to a legacy identity store inside the tunnel.Compare EAP types
EAPOLWireless Auth
EAP over LAN: the frames used between client and AP before the port is authorized. Until authentication passes, EAPOL is all the client may send.Step through EAPOL
Effective Isotropic Radiated PowerEIRPRF & Wireless
Transmit power plus antenna gain minus cable and connector losses — the number regulators cap.Build a gain/loss chain
Equal-cost multipathECMPOSPF
Installing several paths of identical cost for one destination and load-sharing traffic across them.Cost & SPF
Extensible Authentication ProtocolEAPWireless Auth
A container rather than a single method. The chosen EAP type decides what is actually proven: a certificate, a password inside a tunnel, or both.Compare EAP types

F

Fade marginRF & Wireless
Received level minus receiver sensitivity — the headroom protecting the link against rain, multipath and interference. Aim for 15–20 dB.Check your fade margin
Free-space path lossFSPLRF & Wireless
The signal loss from spreading over distance in clear space. It grows with both distance and frequency: 32.44 + 20log(km) + 20log(MHz).Calculate path loss
Frequency bandRF & Wireless
The slice of spectrum a link uses. 2.4 GHz travels further but is crowded; 5 and 6 GHz offer more channels with more path loss.Compare bands in the planner
Fresnel zoneRF & Wireless
The ellipse around the line of sight where obstructions cause interference. Keeping about 60% of the first zone clear is the usual target.Check Fresnel clearance

G

Group temporal keyGTKWireless Auth
The shared key protecting broadcast and multicast frames on a BSS, delivered in message 3 of the 4-way handshake and rotated by the AP.Step through the handshake

H

Hello / dead intervalOSPF
How often OSPF hellos are sent and how long silence is tolerated before a neighbor is declared down. Both must match between neighbors.Single area
Hexadecimal (base 16)hexNumber Bases
Base-16 digits 0–9 and A–F, where one digit is exactly four bits. Used for MAC addresses, IPv6, and protocol header dumps.Convert hex to binary

I

Identity PSKiPSK / PPSKWireless Auth
Per-device passphrases on a single SSID. RADIUS returns the right key for that MAC address before the 4-way handshake runs.See the iPSK lookup
Interior vs exterior gateway protocolIGP / EGPGeneral
IGPs like OSPF find best paths inside one autonomous system; EGPs like BGP exchange reachability between autonomous systems.OSPF Explorer
IPv4 addressAddressing & Subnetting
A 32-bit host identifier written as four decimal octets, such as 192.168.10.25. Part of it identifies the network and part the host.Subnetting Lab
IPv6 addressAddressing & Subnetting
A 128-bit address written as eight hex groups, with runs of zeros compressed to '::'. /64 is the usual subnet size.Expand an IPv6 address

J

JitterGeneral
Variation in packet-to-packet delay. Even with low average latency, high jitter breaks up voice and video and causes buffer underruns.Link quality estimator

L

LatencyGeneral
The time a packet takes to travel from sender to receiver. Lower is better for interactive use; bulk transfer tolerates more.Link quality estimator
Layer 2 vs Layer 3General
Layer 2 switches frames within a broadcast domain using MAC addresses; Layer 3 routers forward packets between networks using IP addresses.
Line of sightLoSRF & Wireless
A clear straight path between antennas. Radio needs Fresnel clearance too, not just a visually unobstructed view.Link planner
Longest prefix matchAddressing & Subnetting
The forwarding rule: when several routes match a destination, the most specific (longest) prefix is used.Subnetting Lab
Loopback addressAddressing & Subnetting
An address that always refers to the device itself — 127.0.0.1 in IPv4, ::1 in IPv6. Also a stable virtual interface on routers.
LSA Type 1 (Router LSA)OSPF
Generated by every router to describe its own interfaces and costs. Flooded only within its own area.LSA types 1–7
LSA Type 2 (Network LSA)OSPF
Generated by the designated router on a multi-access segment to list which routers are attached to it. Area-local.LSA types 1–7
LSA Type 3 (Summary LSA)OSPF
Generated by an area border router to advertise prefixes from one area into another, without the internal topology detail.LSA types 1–7
LSA Type 4 (ASBR Summary)OSPF
Tells routers in other areas how to reach the autonomous system boundary router that injected external routes.LSA types 1–7
LSA Type 5 (External LSA)OSPF
Describes a route redistributed from outside OSPF. Flooded through the whole domain except stub-type areas.LSA types 1–7
LSA Type 7 (NSSA External)OSPF
An external route injected inside a not-so-stubby area. The area border router converts it to a Type 5 on the way out.LSA types 1–7

M

MAC addressNumber Bases
A 48-bit Layer 2 hardware address in hex, split into a vendor prefix (OUI) and a device-specific half.Break down a MAC address
MAC authentication bypassMABWireless Auth
Using the MAC address as the credential for devices that cannot run 802.1X. Useful for inventory control, but trivially spoofed and broken by MAC randomization.See a MAB lookup
Maximum Transmission UnitMTUGeneral
The largest packet size a link carries without fragmenting. Mismatched MTUs cause hard-to-spot failures.
MetricGeneral
The number a routing protocol uses to rank paths. Each protocol defines its own, so metrics are not comparable across protocols.Cost & SPF
Most / least significant bitMSB / LSBNumber Bases
The leftmost bit carries the largest place value (128 in an octet); the rightmost carries the smallest (1).Number Bases
Multi-user MIMOMU-MIMOMU-MIMO
One transmission carrying different data to several clients at the same time, each on its own spatial stream, separated by the direction the AP steers them.MU-MIMO Explorer
MultipathRF & Wireless
Copies of the same signal arriving via reflections at slightly different times, which can reinforce or cancel the direct signal.RF Math Explorer

N

Network addressAddressing & Subnetting
The first address in a subnet, with all host bits zero. It names the subnet and is not assigned to a host.Subnetting Lab
Network Address TranslationNATAddressing & Subnetting
Rewriting addresses (and often ports) at a boundary so many private hosts can share public addresses.
Next hopGeneral
The neighboring router a packet is handed to on its way to the destination.
NibbleNumber Bases
Four bits — exactly one hexadecimal digit. Handy when reading hex as binary.Number Bases
Noise floorRF & Wireless
The background RF energy present regardless of your signal. A high noise floor eats into usable margin.RF Math Explorer
Not-so-stubby areaNSSAOSPF
A stub area that is still allowed to originate its own external routes, carried as Type 7 LSAs.LSA types 1–7
Null data packetNDPMU-MIMO
A frame with no payload, sent on every radio chain purely so clients can measure the channel. It follows an announcement (NDPA) naming who should listen.See where the NDP fits

O

Octal (base 8)Number Bases
Base-8 numbers, where one digit is three bits. Mostly seen in file permissions rather than addressing.Number Bases
Open Shortest Path FirstOSPFOSPF
A link-state interior routing protocol. Every router floods a description of its own links, builds an identical map of the area, and runs Dijkstra to pick shortest paths.OSPF Explorer
Opportunistic Wireless EncryptionOWEWireless Auth
Enhanced Open: a Diffie-Hellman exchange during association gives every client on an open SSID its own encryption keys, with no credential involved.Run an OWE exchange
Organizationally Unique IdentifierOUINumber Bases
The first 24 bits of a MAC address, assigned to a manufacturer. Useful for identifying who made a device.Break down a MAC address
Orthogonal Frequency-Division Multiple AccessOFDMAMU-MIMO
Splitting a channel into smaller resource units so several clients share it in frequency. Complementary to MU-MIMO, which shares it in space.Compare with spatial sharing
OSI modelGeneral
A seven-layer reference model, from physical cabling up to applications. Handy for placing where a protocol or problem lives.
OSPF costOSPF
The metric OSPF adds up along a path — traditionally reference bandwidth divided by interface bandwidth. Lower total cost wins.Cost & SPF
Outer and inner identityWireless Auth
The outer identity travels in the clear and is often anonymous; the real username is the inner identity, sent only inside the TLS tunnel.See where identity is exposed

P

Packet lossGeneral
The fraction of packets that never arrive. A small percentage compounds quickly through retransmissions and codec concealment.Link quality estimator
Pairwise master keyPMKWireless Auth
The seed both sides derive independently — from the passphrase, from SAE, or from the EAP exchange — and then prove they hold during the 4-way handshake.See where the PMK comes from
Pairwise transient keyPTKWireless Auth
The session key that actually encrypts unicast traffic, built from the PMK plus both nonces and both MAC addresses, so it is unique per association.Step through the handshake
Parabolic dish gainRF & Wireless
Approximate gain from dish diameter, frequency and aperture efficiency — bigger dishes and higher frequencies give more gain.Estimate dish gain
Path costSpanning Tree
The accumulated cost of the links between a switch and the root bridge. Faster links cost less, so the cheapest total path is chosen.Spanning Tree Explorer
PEAP-MSCHAPv2Wireless Auth
The most common enterprise setup: a TLS tunnel authenticating only the server, with a username and password checked inside it. Safe only when clients validate the server certificate.Run a PEAP exchange
Place valueNumber Bases
The weight of each digit position: 128 64 32 16 8 4 2 1 across an octet. Converting is just adding the weights that are set.Number Bases
Port numberNumber Bases
A 16-bit value (0–65535) identifying a service or session endpoint, often quoted in hex when reading packet captures.Number Bases
Prefix length / CIDR notationCIDRAddressing & Subnetting
Shorthand for the mask, written as a slash and the count of network bits: 10.0.0.0/24 means 24 network bits and 8 host bits.Subnetting Lab
Private address spaceAddressing & Subnetting
Ranges reserved for internal use — 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 — that are not routed on the public internet.Subnetting Lab
Protected Management FramesPMFWireless Auth
802.11w protection for management traffic, which blocks forged deauthentication and disassociation attacks. Optional under WPA2, mandatory under WPA3 and OWE.Toggle PMF and see the effect

R

Radio chainMU-MIMO
A transmit/receive path with its own antenna. A 4x4 radio has four, and the chain count caps how many spatial streams it can send at once.Set AP chains and see the limit
RADIUSWireless Auth
The protocol carrying authentication requests to a server on UDP 1812 (1813 for accounting). Packets are signed with a shared secret agreed between the server and each network device.Watch RADIUS in the flow
Rapid Spanning Tree ProtocolRSTPSpanning Tree
The faster successor to classic STP (802.1w). It reconverges in a second or two by negotiating with neighbors instead of waiting out fixed timers.Watch a link fail and reconverge
Received signal levelRSSIRF & Wireless
The power arriving at the receiver, in dBm: EIRP plus receive antenna gain minus path loss and receive-side losses.Link planner
Receiver sensitivityRF & Wireless
The weakest signal a radio can still decode at a given data rate. Faster rates need stronger signals.Link planner
RedistributionOSPF
Importing routes from one routing protocol into another. Metrics do not translate directly, so the imported cost must be chosen deliberately.OSPF ↔ BGP redistribution
Root bridgeSpanning Tree
The single switch elected as the reference point of the tree. Every other switch calculates its best path toward the root, and all of the root's ports forward.Change priorities and re-elect the root
Root portSpanning Tree
The one port on a non-root switch with the lowest cost path toward the root bridge. It always forwards.See port roles on the topology
Router IDRIDOSPF
The 32-bit identifier a router uses in OSPF, written like an IP address. It names the router in every LSA it originates.OSPF Explorer
Routing tableGeneral
The list of destination prefixes a router knows, with the next hop, exit interface and metric for each.Cost & SPF

S

Shortest Path First (Dijkstra)SPFOSPF
The algorithm each router runs over its link-state database to compute the lowest-cost tree from itself to every destination.Cost & SPF
Signal-to-noise ratioSNRRF & Wireless
How far the wanted signal sits above the noise floor, in dB. It, more than raw signal strength, determines achievable data rate.RF Math Explorer
Simultaneous Authentication of EqualsSAEWireless Auth
The WPA3-Personal exchange. A commit and confirm pair proves both sides know the passphrase without exposing anything an attacker could crack offline.Run an SAE exchange
Single-user MIMOSU-MIMOMU-MIMO
Multiple spatial streams all sent to one client. A 1x1 client on a big AP leaves most radio chains idle, which is the waste MU-MIMO recovers.Compare SU and MU airtime
Spanning Tree ProtocolSTPSpanning Tree
A switch protocol that blocks redundant links so a Layer 2 network has exactly one active path between any two points, preventing broadcast loops.Spanning Tree Explorer
Spatial correlationMU-MIMO
How similar two clients' channels look to the AP. Highly correlated clients cannot be separated cleanly, so grouping them buys little or nothing.See correlation cost the gain
Spatial streamMU-MIMO
One independent data stream carried over the same channel at the same time, separated from the others by its spatial path rather than by frequency.Allocate streams to clients
Steering matrixMU-MIMO
The set of per-antenna weights the AP computes from client feedback, used to aim each spatial stream at its client while nulling it toward the others.Build a steering matrix
Stub areaOSPF
An area that refuses external LSAs and relies on a default route from its border router, keeping its database small.Multi-area design
Subnet maskAddressing & Subnetting
A 32-bit pattern of leading ones that marks which bits of an address are the network portion — for example 255.255.255.0.Subnetting Lab
Supernetting / route summarizationAddressing & Subnetting
Combining adjacent prefixes into one shorter prefix so routers advertise fewer, larger routes.Subnetting Lab
SupplicantWireless Auth
The software on the client that carries out the authentication exchange. Its profile decides which EAP method is used and which server certificate is trusted.See the roles in the exchange

T

ThroughputGeneral
The useful data actually delivered per second — not the same as link speed. Loss, retransmissions, and overhead all reduce it.Link quality estimator

U

Usable host rangeAddressing & Subnetting
The addresses between the network and broadcast addresses — 2^(host bits) − 2 of them in ordinary IPv4 subnets.Subnetting Lab

V

Variable Length Subnet MaskingVLSMAddressing & Subnetting
Carving one block into subnets of different sizes so each segment gets only the addresses it needs, largest allocated first.Plan a VLSM allocation
VLANSpanning Tree
A virtual LAN — a logically separate Layer 2 broadcast domain carried over shared switch hardware and trunk links.

W

WavelengthλRF & Wireless
The physical length of one cycle, roughly 300 divided by the frequency in MHz (metres). Antenna sizes follow from it.Calculate wavelength
Wildcard maskAddressing & Subnetting
The bitwise inverse of a subnet mask, used in access lists and OSPF network statements. 0 means match, 1 means ignore.Subnetting Lab
WPA3 transition modeWireless Auth
One SSID advertising both SAE and PSK so older clients still join. Convenient, but those clients keep WPA2's offline passphrase exposure.See what transition mode costs

Z

Zero compression (IPv6)Number Bases
Replacing one run of all-zero IPv6 groups with '::'. It may appear only once per address so it can be expanded unambiguously.Expand an IPv6 address