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Glossary

Short definitions, in alphabetical order, each pointing to where the term shows up in the tools.

Adapter
Windows' word for a network interface: the Ethernet port, the Wi-Fi radio, or a virtual device such as a VPN tunnel. Each has a connection name ("Ethernet", "Wi-Fi") used in ipconfig /renew "Wi-Fi".
Anycast
One address announced from many places; the network delivers to the nearest. How 1.1.1.1 and 8.8.8.8 work. Invisible in local tools.
APIPA
Automatic Private IP Addressing. Windows' name for self-assigning a 169.254.x.x address when DHCP fails. Appears as Autoconfiguration IPv4 Address.
ARP
Address Resolution Protocol. Maps an IPv4 address to a MAC address on the local segment. The cache is shown by arp -a, ip neigh, arp -an.
Broadcast address
The address that reaches every host on a subnet: all host bits set to 1, e.g. 192.168.1.255 for 192.168.1.0/24. ifconfig shows it as broadcast or Bcast.
CGNAT
Carrier-grade NAT. The ISP shares one public IPv4 address among many customers using 100.64.0.0/10 internally. Breaks inbound connections and port forwarding.
CIDR
Classless Inter-Domain Routing notation: an address followed by a slash and the number of network bits, e.g. 192.168.1.0/24. Equivalent to a subnet mask. Table →
Default gateway / default route
Where packets go when no more specific route matches: your router. ipconfig shows Default Gateway; ip route shows default via.
DHCP
Dynamic Host Configuration Protocol. A server hands out addresses, masks, gateways and DNS servers as time-limited leases. Managed by ipconfig /release and /renew, dhclient, ipconfig set en0 DHCP.
DHCPv6
DHCP for IPv6. Stateful DHCPv6 assigns addresses; stateless DHCPv6 supplies only DNS and other options alongside SLAAC. ipconfig /renew6 drives it.
DNS
Domain Name System. Translates names to addresses. Resolver addresses show in ipconfig /all, resolvectl status, scutil --dns.
DNS suffix
A domain appended to short names before lookup. Connection-specific comes from DHCP; Primary from domain membership; the search list is tried in order.
DoH / DoT
DNS over HTTPS / DNS over TLS. Encrypted DNS. Configured per browser or per OS (Windows 11, Android Private DNS, systemd-resolved DNSOverTLS=).
DUID
DHCP Unique Identifier. A per-machine identifier used by DHCPv6, shown as DHCPv6 Client DUID. The per-interface part is the IAID.
Duplex
Whether a link can send and receive simultaneously (full) or not (half). Half-duplex on a switched network is a fault. Shown by ethtool, ifconfig media line on BSD.
EUI-64
A way of building a 64-bit IPv6 interface identifier from a 48-bit MAC by inserting ff:fe in the middle and flipping one bit. Recognisable by ff:fe in the middle of the address. Being replaced by random identifiers for privacy.
Flags
Bit-field describing interface state: UP, RUNNING, BROADCAST, MULTICAST, PROMISC, LOWER_UP, NO-CARRIER. Table →
Happy Eyeballs
RFC 8305. Browsers race IPv6 and IPv4 connections so a broken IPv6 path does not cause long delays. Command-line tools mostly do not implement it.
Hosts file
A static name-to-address table consulted before DNS: C:\Windows\System32\drivers\etc\hosts, /etc/hosts.
ifindex / interface index
The kernel's integer for an interface. The number before the name in ip addr, and the zone ID in Windows link-local addresses (%12).
Interface
A network attachment point: physical (NIC, Wi-Fi) or virtual (loopback, tunnel, bridge, VLAN). Windows says adapter; macOS says port or service.
Lease
A DHCP assignment with a start and end time. Clients renew at half-life. Shown as Lease Obtained / Expires or valid_lft.
An address valid only on the local segment: 169.254.0.0/16 for IPv4, fe80::/10 for IPv6. Every IPv6 interface has one automatically.
LLMNR / mDNS
Link-Local Multicast Name Resolution (Windows) and multicast DNS (Bonjour, Avahi). Resolve names on the local network without a DNS server; mDNS handles the .local domain.
Loopback
The virtual interface that talks to the same machine: 127.0.0.1 and ::1, named lo or lo0.
MAC address
Media Access Control address: the 48-bit hardware address of an Ethernet or Wi-Fi interface. Called Physical Address on Windows, ether or HWaddr on Unix. Formats and OUIs →
Metric
A route's cost; lower wins when routes tie. How Windows and NetworkManager choose between Ethernet and Wi-Fi.
MTU
Maximum Transmission Unit: the largest packet an interface sends, in bytes. 1500 for Ethernet, 1492 for PPPoE, 9000 for jumbo frames. Problems →
Multicast
One-to-many delivery to subscribed hosts. 224.0.0.0/4 and ff00::/8. Used by mDNS, SSDP, IPv6 neighbour discovery, streaming.
NAT
Network Address Translation. A router rewrites private source addresses to its own public address. The reason your public and local IP differ. More →
NDP
Neighbour Discovery Protocol: IPv6's replacement for ARP, plus router discovery. Cache shown by ip -6 neigh, ndp -an, netsh interface ipv6 show neighbors.
NetBIOS
A 1980s naming and session API kept alive by Windows file sharing. NetBIOS over Tcpip and Node Type in ipconfig /all refer to it. Mostly harmless legacy.
net-tools
The Linux package containing ifconfig, route, arp, netstat. Deprecated in favour of iproute2.
Netmask / subnet mask
A bit-mask marking the network part of an address: 255.255.255.0 = /24. BSD prints it in hex (0xffffff00).
NIC
Network interface card, or controller. The hardware behind an interface.
OUI
Organisationally Unique Identifier: the first three bytes of a MAC address, assigned to a manufacturer.
Prefix length
The CIDR number of network bits, e.g. /64 for an IPv6 subnet. Shown as prefixlen by ifconfig and as the suffix by ip.
Promiscuous mode
An interface accepting all frames on the segment, not just its own. Set by packet capture tools; shows as PROMISC.
Route
An entry saying "to reach network X, send via gateway Y on interface Z". The routing table is shown by route print, ip route, netstat -rn.
The Linux kernel socket interface used by ip and NetworkManager to read and change network state. ifconfig uses the older ioctl interface.
SLAAC
Stateless Address Autoconfiguration: an IPv6 host builds its own address from the router's advertised prefix. No server needed. The source of most home IPv6 addresses.
Temporary address
A randomised, short-lived IPv6 address used for outgoing connections to limit tracking (RFC 8981). Rotates roughly daily.
TTL
Time To Live. For DNS, how long an answer may be cached; shown in ipconfig /displaydns and dig. For IP packets, a hop limit decremented by each router; how traceroute works.
ULA
Unique Local Address: fc00::/7 (in practice fd00::/8), the IPv6 equivalent of private addresses.
Unicast / global unicast
One-to-one addressing. A global unicast IPv6 address (2000::/3) is publicly routable.
VLAN
Virtual LAN: a tagged Ethernet segment (802.1Q). Appears as eth0.10 on Linux, vlan10 on BSD.
WINS
Windows Internet Name Service, the NetBIOS name server. Obsolete; the WINS Proxy Enabled line in ipconfig /all is a fossil.
Zone ID / scope ID
The %12 or %eth0 suffix on a link-local IPv6 address, naming the interface it belongs to.

Related pages

Last reviewed . Command syntax verified against Windows 11, Ubuntu 24.04, macOS 15 and FreeBSD 14 unless noted otherwise.

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