Network engineers and application developers don’t always share vocabulary. You read about an outage caused by “a BGP route leak from a peer to a transit provider that bypassed the IRR-validated prefix filter” and most of those words mean something specific. This post is a working glossary — 60 terms that come up in modern infrastructure work, with practical definitions and links to deeper coverage where available.
A — F
Anycast — One IP advertised from many physical locations; users reach the nearest. Used by every major CDN and DNS resolver. Anycast vs unicast.
ASN — Autonomous System Number. Identifies a network operator in BGP routing. What is an ASN.
AS Path — In BGP, the sequence of ASNs a route traversed. Helps detect loops and pick shortest paths.
ARIN — American Registry for Internet Numbers. The RIR covering North America.
AAAA Record — DNS record for an IPv6 address (compare to A for IPv4).
BGP — Border Gateway Protocol. The routing protocol that holds the internet together. BGP routing explained.
Broadcast — A packet sent to all hosts on a network. IPv4 has broadcast addresses; IPv6 replaces this with multicast.
CGNAT — Carrier-Grade NAT. ISP-scale NAT that puts thousands of subscribers behind one public IP. NAT and CGNAT.
CIDR — Classless Inter-Domain Routing. Notation like 192.168.0.0/24 for IP subnets. Subnet mask CIDR explained.
CDN — Content Delivery Network. Distributed caches that serve content from edges close to users. CDN vs edge computing.
Container CNI — Container Network Interface. Plugin spec for assigning IPs to containers (Calico, Flannel, Cilium).
Default route — 0.0.0.0/0 (IPv4) or ::/0 (IPv6). “If you don’t know where to send this, send it here.”
DHCP — Dynamic Host Configuration Protocol. Hands out IPs and network config. DHCP explained.
DNS — Domain Name System. Maps names to IPs. How DNS lookup works.
DPI — Deep Packet Inspection. Examining packet payloads, not just headers. Used by firewalls, ISPs (for shaping), and surveillance.
ECMP — Equal-Cost Multi-Path. Load-balancing across multiple routes with the same metric. Used inside data centers.
ECH — Encrypted Client Hello. Encrypts the SNI field in TLS so on-path observers can’t see which site you’re connecting to.
Edge — A POP or location near users where compute or caching happens. CDN vs edge computing.
Egress — Traffic leaving a network (or VPC).
eBGP / iBGP — External BGP (between ASes) vs internal BGP (within an AS).
Encapsulation — Wrapping one packet inside another (VLAN, VXLAN, GRE, IPsec).
Firewall — Filters traffic by rules. Stateful (tracks connections) vs stateless.
G — N
Gateway — A router that connects networks. Usually the device that forwards your traffic toward the wider internet.
GeoDNS — DNS that returns different records based on the querier’s location.
HSRP / VRRP — Protocols for active/standby router failover.
ICMP — Internet Control Message Protocol. Used for ping, traceroute, error messages. ICMP, ping, and traceroute.
Ingress — Traffic entering a network. Also a Kubernetes resource for HTTP routing.
IRR — Internet Routing Registry. Database of declared routing intentions; used to filter BGP advertisements.
IPS / IDS — Intrusion Prevention/Detection System. Watches network traffic for attacks.
IPv4 — Internet Protocol version 4. 32-bit addresses. The dominant version. Backbone of the internet.
IPv6 — Internet Protocol version 6. 128-bit addresses. The future. IPv6 the future.
ISP — Internet Service Provider. The company that connects you to the internet.
IX / IXP — Internet Exchange Point. Where networks peer with each other. Internet exchange points.
Jumbo frame — Ethernet frame larger than the default 1500 MTU (typically 9000). Used inside data centers.
Latency — Time for a packet to travel one direction. Often confused with round-trip time.
LAN — Local Area Network. The network at your home or office.
Layer 2 / Layer 3 — OSI model. L2 = data link (Ethernet, MAC addresses). L3 = network (IP, routing).
LIR — Local Internet Registry. Usually an ISP that gets allocations from an RIR. IP allocation explained.
Loopback — 127.0.0.1 (IPv4) or ::1 (IPv6). The device’s own address; never leaves the host.
MAC address — Hardware identifier for a network interface. 48 bits. Layer 2.
Mbps / Gbps — Megabits/Gigabits per second. Throughput unit.
MPLS — Multiprotocol Label Switching. ISP backbone tech for tunneling and traffic engineering.
MTU — Maximum Transmission Unit. Largest packet a link can carry. MTU and MSS.
Multicast — One sender, many receivers via group address. Used for video, IPv6 ND.
NAT — Network Address Translation. Maps internal private IPs to external public IPs. NAT and CGNAT.
NTP — Network Time Protocol. Synchronizes clocks across machines.
O — Z
OOB — Out of Band. Management traffic on a separate network from production.
OSPF — Open Shortest Path First. Interior routing protocol (within an AS).
Overlay — A virtual network built on top of physical infrastructure (VXLAN, Geneve).
Peering — Two networks exchanging traffic directly (vs through a transit provider). Internet exchange points.
PMTUD — Path MTU Discovery. Mechanism for finding the smallest MTU on a path. MTU and MSS.
POP — Point of Presence. Physical location where a network has equipment (often a data center).
PPPoE — PPP over Ethernet. Used by some DSL ISPs to deliver IPs.
Private IP — RFC 1918 ranges (10/8, 172.16/12, 192.168/16). Not internet-routable. Private IP ranges.
QoS — Quality of Service. Prioritizing certain traffic over others.
QUIC — UDP-based transport with TLS built in. The transport under HTTP/3. HTTP/2 vs HTTP/3.
Reverse DNS / PTR — Mapping from IP back to a hostname. Reverse DNS PTR.
RIR — Regional Internet Registry. ARIN, RIPE, APNIC, LACNIC, AFRINIC. IP allocation.
Route flap — A route repeatedly going up and down. Causes BGP instability.
RPKI — Resource PKI. Cryptographic validation of BGP route origins. BGP hijacking.
SDN — Software Defined Networking. Network behavior controlled by programs, not box-by-box config.
SNI — Server Name Indication. TLS extension that says which hostname the client wants. TLS handshake.
Spine-Leaf — Modern data center topology. Every leaf switch connects to every spine switch.
STP — Spanning Tree Protocol. Prevents loops in L2 networks.
Subnet — A range of IP addresses sharing a prefix. Subnetting cheatsheet.
TCP — Transmission Control Protocol. Reliable, ordered transport. TCP vs UDP.
Tier 1 / Tier 2 / Tier 3 — Informal classification of ISPs by global reach. Tier 1s don’t pay for transit anywhere.
TLS — Transport Layer Security. Encrypts TCP connections. TLS handshake.
Transit — Paying an upstream ISP to carry your traffic globally. Vs peering, which is free.
TTL — Time To Live. (1) IP header field decremented at each hop. (2) DNS record duration in cache.
UDP — User Datagram Protocol. Unreliable, lightweight transport. TCP vs UDP.
Unicast — One sender, one receiver. The default. Anycast vs unicast.
VLAN — Virtual LAN. L2 segmentation using tags.
VPC — Virtual Private Cloud. Logically isolated network in a cloud provider.
VPN — Virtual Private Network. Encrypted tunnel; makes you appear as if on a different network.
VRF — Virtual Routing and Forwarding. Multiple independent routing tables on one device.
VXLAN — VLAN-like L2 overlay over IP. Used inside data centers.
WAN — Wide Area Network. Spans larger geographic areas than a LAN.
WAF — Web Application Firewall. Filters HTTP traffic by application-layer rules.
X-Forwarded-For — HTTP header carrying the original client IP through proxies. X-Forwarded-For.
Zero Trust — Security model where every connection is authenticated and authorized, not just trusted by network position.
Where Each Class of Term Fits
A simplified map:
- Addressing & naming: IP, DNS, ASN, CIDR, RIR, LIR, MAC.
- Routing: BGP, OSPF, AS Path, Default route, Anycast, RPKI.
- Transport: TCP, UDP, QUIC, TLS, MTU, PMTUD.
- NAT & private networks: NAT, CGNAT, RFC 1918, VPC, Subnet.
- Performance & topology: ECMP, POP, IX, Tier 1, Spine-Leaf.
- Operations: DHCP, NTP, ICMP, traceroute, SNI.
- Security: Firewall, WAF, IDS, VPN, Zero Trust, ECH.
- Application proxies: X-Forwarded-For, CDN, Edge, GeoDNS.
TL;DR
A working network vocabulary for application developers in 2026 includes a few dozen terms. Most fit into a small number of categories: addressing, routing, transport, security, operations. The links above lead to deeper coverage where the term warrants its own post.
If you remember nothing else from this glossary: when reading any network-related incident report, the most important terms are usually BGP (something about routing), ASN (who owns the network), and CIDR (which addresses are affected). The rest is detail.
For broader IP knowledge, see everything you need to know about IP addresses; for the ASN level, what is an ASN. To put many of these terms into practice via API, the Ip2Geo API returns the ASN, geolocation, and network classification with every lookup.