Web Uptime & Infrastructure Glossary
Plain-English explanations of 61 technical terms used in website monitoring, DNS, CDNs, and infrastructure.
A
- API
- Application Programming Interface. A set of rules that lets one software program talk to another. When a website uses an API, it's fetching data from a remote server via structured requests, usually in JSON format.
- AWS
- Amazon Web Services. The world's largest cloud computing platform, used by a significant portion of the internet for hosting, storage, and infrastructure.
B
- Backend
- The server-side of a web application — databases, application logic, and data processing. Users don't see the backend directly; they interact with the frontend.
- Bad Gateway (502)
- An HTTP error indicating that a proxy or load balancer received an invalid response from an upstream server. Usually means the backend application has crashed.
- Bandwidth
- The maximum amount of data that can be transmitted over a network in a given time. Measured in bits per second (bps, Mbps, Gbps).
C
- CDN
- Content Delivery Network. A network of servers worldwide that cache and deliver website content to users from the geographically nearest server, reducing latency. Major CDNs: Cloudflare, Akamai, Fastly, AWS CloudFront.
- Cloudflare
- A major CDN and DDoS-protection provider. A large portion of the web sits behind Cloudflare. When Cloudflare has issues, many sites appear down simultaneously.
- Connection Timeout
- When a client (browser or server) can't establish a TCP connection to a target server within a given time window. Indicates the target isn't responding at all.
D
- DDoS
- Distributed Denial of Service. An attack where thousands (or millions) of compromised devices flood a target server with requests, overwhelming it and making the service unavailable to legitimate users.
- DNS
- Domain Name System. The internet's phone book — translates human-readable domain names (like example.com) to IP addresses that computers use. When DNS fails, websites appear unreachable even though the server is fine.
- DNS_PROBE_FINISHED_NXDOMAIN
- A Chrome error meaning your DNS server can't find the domain. Either the domain doesn't exist, has expired, or your DNS is misconfigured.
- Downtime
- Any period during which a service is unavailable. Measured in minutes per month or expressed as a percentage (e.g., 99.9% uptime = ~43 minutes of downtime per month).
E
- Edge Server
- A CDN server located geographically close to users. Edge servers cache content to reduce the time it takes to deliver data. Part of what makes big sites fast.
F
- Favicon
- The small icon you see in browser tabs and bookmarks. Also used by nere.nu as a quick way to probe whether a site is reachable from your browser.
- Firewall
- Network security system that filters incoming and outgoing traffic based on rules. Can be hardware (router) or software (on your computer).
- Forbidden (403)
- HTTP status code meaning the server understands the request but refuses to fulfill it. Often caused by WAF rules, geo-blocking, or authentication requirements.
- Frontend
- The client-side of a web application — what users see and interact with in their browser. Usually built with HTML, CSS, and JavaScript.
G
- GDPR
- General Data Protection Regulation. EU law protecting personal data. Requires transparent data handling, user rights to access/delete data, and explicit consent for non-essential tracking.
- Gateway Timeout (504)
- HTTP error where a proxy or CDN waited too long for a response from the origin server. Usually means the backend is overloaded or has crashed.
H
- HSTS
- HTTP Strict Transport Security. A header that tells browsers to always use HTTPS for a domain. Protects against downgrade attacks. Shown in our Infrastructure panel.
- HTTP
- HyperText Transfer Protocol. The foundation of data communication on the web. When you visit a website, your browser sends HTTP requests and receives HTTP responses.
- HTTP/2
- A modern version of HTTP that allows multiple parallel requests over a single connection, reducing load times. Widely supported by modern servers and browsers.
- HTTP/3
- The latest HTTP version, built on QUIC instead of TCP. Offers faster connection establishment and better performance on unreliable networks.
- HTTPS
- HTTP Secure. HTTP encrypted with TLS. Protects data in transit from being read or modified by attackers. The padlock in your browser's address bar indicates HTTPS.
I
- IP Address
- Internet Protocol address. A unique numerical identifier for a device on a network. IPv4 addresses look like 192.168.1.1; IPv6 addresses are longer (e.g., 2001:db8::1).
- ISP
- Internet Service Provider. The company providing your internet connection (Comcast, Verizon, Bahnhof, etc.). ISPs can affect your ability to reach specific sites through routing decisions.
- Internal Server Error (500)
- A generic HTTP error meaning the server crashed or malfunctioned without providing specific details. Could be a code bug, database failure, or configuration error.
L
- Latency
- The time delay between a request and a response. Measured in milliseconds. Physical distance, network congestion, and server processing time all contribute to latency.
- Load Balancer
- A system that distributes incoming traffic across multiple servers to prevent any single server from being overwhelmed. Critical for high-traffic sites.
M
- MTTR
- Mean Time To Repair. The average time it takes to fix a failure and restore service. A key reliability metric.
- Monitoring
- The practice of continuously checking whether a system is functioning correctly. Uptime monitoring (what nere.nu does) specifically tracks whether a website is accessible.
N
- Nameserver
- A server that provides DNS resolution. Large organizations run their own (e.g., ns1.cloudflare.com); smaller sites use their registrar's nameservers.
- Not Found (404)
- HTTP error meaning the requested resource doesn't exist on the server. The server itself is working fine; the specific URL just isn't valid.
O
- Origin Server
- The main server where a website's content actually lives. CDNs cache content from the origin server to deliver it faster to users.
- Outage
- A period of unplanned downtime. Can be local (affecting one user), regional (one ISP or geographic area), or global (the entire service is down).
P
- Packet Loss
- When data packets fail to reach their destination during transmission. Small amounts are normal; high packet loss makes connections slow or unusable.
- Ping
- A network utility that tests whether a host is reachable by sending small data packets and measuring response time. Different from an HTTP check — ping works at a lower network layer.
- Port
- A numbered endpoint on a server for network communication. HTTP uses port 80, HTTPS uses port 443. Some services run on custom ports.
- Proxy
- A server that acts as an intermediary between clients and other servers. Used for caching, filtering, load balancing, or anonymity.
Q
- QUIC
- A modern transport protocol (built on UDP) that underpins HTTP/3. Offers faster handshakes and better performance than TCP on lossy networks.
R
- Redirect (301/302)
- HTTP responses that tell browsers to go to a different URL. 301 = permanent, 302 = temporary. Redirects can chain, which adds latency.
- Response Time
- The time from sending a request to receiving the complete response. Includes DNS lookup, TCP connection, TLS handshake, and data transfer.
- Reverse Proxy
- A proxy that sits in front of web servers, forwarding requests from clients. Often provides SSL termination, caching, and load balancing. Nginx and HAProxy are common reverse proxies.
S
- SSL Certificate
- A digital certificate issued by a Certificate Authority (CA) that authenticates a website's identity and enables encrypted connections. Let's Encrypt is the most popular free CA.
- SSL/TLS
- Secure Sockets Layer / Transport Layer Security. Cryptographic protocols that encrypt traffic between clients and servers. Modern HTTPS uses TLS (TLS 1.2 or 1.3).
- SSRF
- Server-Side Request Forgery. A vulnerability where an attacker tricks a server into making requests to internal systems. nere.nu prevents SSRF by blocking checks to private IP ranges.
- Server
- A computer that provides services to other computers (clients). Can refer to physical hardware, virtual machines, or software processes.
- Service Unavailable (503)
- HTTP error meaning the server is temporarily unable to handle requests. Common during maintenance, overload, or deployments.
- Status Code
- A 3-digit number in HTTP responses indicating the outcome of a request. 2xx = success, 3xx = redirect, 4xx = client error, 5xx = server error.
- Subdomain
- A prefix to a domain name (e.g., blog.example.com, api.example.com). Often used to separate services or environments.
T
- TCP
- Transmission Control Protocol. A reliable, ordered network protocol used by HTTP/HTTPS. Guarantees that data arrives in order and without loss.
- TLD
- Top-Level Domain. The last part of a domain name (.com, .nu, .org). There are generic TLDs (gTLDs) and country-code TLDs (ccTLDs like .se or .nu).
- TLS Handshake
- The process of establishing an encrypted connection. Client and server exchange keys and agree on encryption parameters. Adds latency (one or two extra round trips).
- TTFB
- Time To First Byte. How long from sending a request until the first byte of response arrives. A key performance metric.
- TTL
- Time To Live. How long a piece of data (usually a DNS record) should be cached before being refreshed. Low TTLs mean faster updates, higher TTLs reduce load.
U
- UUID
- Universally Unique Identifier. A 128-bit number used to uniquely identify resources across distributed systems, without coordination.
- Uptime
- The percentage of time a service is available. Common targets: 99% (3.65 days down/year), 99.9% ("three nines", 8.7h/year), 99.99% ("four nines", 52m/year).
V
- VPN
- Virtual Private Network. Encrypts your internet traffic and routes it through a remote server, hiding your IP and bypassing geo-restrictions.
W
- WAF
- Web Application Firewall. Filters malicious HTTP traffic before it reaches the application. Cloudflare and AWS WAF are popular. Sometimes blocks legitimate requests too aggressively.
- Webhook
- An HTTP callback triggered by an event. Instead of polling for updates, the source system pushes data to a URL when something happens.
- www
- The traditional "World Wide Web" prefix for domains. Today, most sites redirect www.example.com to example.com (or vice versa) for canonical URLs.
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