Web and networking glossary

A defined way for one program to ask another for data or actions. Web APIs usually accept HTTP requests and return JSON.

Finds the hardware (MAC) address that belongs to an IPv4 address on the same local network. IPv6 uses Neighbor Discovery for the same job.

A network of servers in many locations that caches copies of a site’s files close to visitors, cutting latency and load on the origin server.

Notation that writes an IP address range as an address plus a prefix length, such as 192.0.2.0/24, where /24 means the first 24 bits identify the network.

A DNS record that makes one hostname an alias of another. A name with a CNAME can’t hold other records, which is why it can’t sit at the zone apex.

Response headers that tell a browser when a page from one origin may read responses from another. It relaxes the same-origin policy; it doesn’t protect the server.

A response header that lists where a page may load scripts, styles and other resources from, limiting what injected code can do.

Adds a cryptographic signature to outgoing email. Receivers check it against a public key published in DNS under a selector.

A DNS policy that tells receivers what to do with mail that fails SPF and DKIM alignment (none, quarantine or reject) and where to send reports.

The distributed system that turns domain names into the records computers use, such as IP addresses and mail server names.

Signs DNS records so a validating resolver can confirm an answer came from the zone and wasn’t altered. It doesn’t encrypt queries.

A response header that tells browsers to use only HTTPS for a site for a set time, blocking downgrades to plain HTTP.

The request-and-response protocol browsers and servers use to exchange pages and API data. Each request stands alone; cookies and tokens add state.

HTTP sent over an encrypted TLS connection. It stops others on the network from reading or changing traffic, but says nothing about whether a site is honest.

The protocol that addresses and routes packets between networks. IPv4 addresses are 32 bits long; IPv6 addresses are 128 bits.

A compact signed token carrying claims, such as who the user is and when the token expires. A server must verify the signature and claims before trusting it.

An open protocol that lets AI applications connect to servers offering tools, data and prompts. A server’s tools decide what a model can reach.

A DNS record naming the servers that accept email for a domain, each with a preference number. Lower numbers are tried first.

Rewrites addresses as traffic passes through a router, so many devices on a private network can share one public IPv4 address.

A standard JSON or YAML format for describing an HTTP API’s endpoints, parameters, schemas and authentication.

A response, usually 301, 302, 307 or 308 with a Location header, that sends the client to a different URL.

An architectural style for web APIs built around resources identified by URLs and handled with standard HTTP methods.

A text file at a site’s root that tells crawlers which paths they may fetch. It controls crawling, not indexing, and it isn’t access control.

Controls whether a cookie is sent with requests that start on other sites: Strict, Lax or None. It is one of the main defenses against CSRF.

A missing page that returns status 200 instead of 404 or 410, so crawlers and link checkers treat it as a working page.

A TXT record listing the servers allowed to send email for a domain. Checking it may take at most 10 DNS lookups.

An attack that tricks a server into requesting a URL the attacker chooses, often to reach internal systems or cloud metadata addresses.

The three-digit number at the start of an HTTP response: 2xx success, 3xx redirect, 4xx client error and 5xx server error.

A connection-oriented transport protocol that delivers a reliable, ordered stream of bytes and resends anything lost.

The protocol that encrypts and authenticates connections such as HTTPS. It replaced SSL, though certificates are still often called SSL certificates.

In DNS, the number of seconds a resolver may cache a record. In an IP packet, a hop counter that stops packets from circulating forever.

A connectionless transport protocol that sends independent datagrams without delivery guarantees. DNS, video calls and QUIC use it.

A web address made of a scheme, host, optional port, path, query string and fragment, for example https://example.com/docs?page=2.

An encrypted tunnel between your device and a VPN server. Your traffic leaves the server as if it started there.

An HTTP request a service sends to a URL you register when an event happens, such as a payment succeeding.

A file listing the URLs you want search engines to discover, with optional last-modified dates.