What are HTTP status codes?

HTTP status codes: definition and meaning

Whenever a browser or a search engine crawler requests a page, the server answers with a status code. It sits in the header of the response and is usually invisible to visitors. For search engines, monitoring, and interfaces it is decisive, because it says how the address should be handled. The codes are defined in the HTTP standard (RFC 9110).

The first digit determines the class, so a code can be categorized even without detailed knowledge: 2s stand for success, 3s for redirects, 4s for errors in the request, and 5s for errors on the server side. The most important codes at a glance:

• 2xx – success: 200 OK
• 3xx – redirect: 301 and 308 permanent, 302 and 307 temporary, 304 not modified
• 4xx – request errors: 401 not authenticated, 403 forbidden, 404 not found, 410 gone, 429 too many requests
• 5xx – server errors: 500 internal error, 502 bad gateway, 503 service unavailable

Status codes from an SEO perspective

Search engines derive actions from the code. With 200, the content can be indexed. With 301 and 308, Google adopts the new address along with its signals; with 404 and 410, the page drops out of the index over time. Persistent 5xx errors make Google crawl less, and pages can drop out of the index.

For planned maintenance, 503 (Service Unavailable), ideally with the Retry-After header, is the right code because it signals a temporary outage. A maintenance page with status 200, by contrast, can lead Google to index the maintenance text and temporarily push the actual content out of the results.

Real-world example: a tax advisory firm moves domains

A tax advisory firm moves to a new domain. Every old address is redirected with a 301 to the topically matching new page, not across the board to the home page. For the database switchover, a short maintenance window is announced with 503 and Retry-After.

After the move, Search Console shows no error spikes, and visibility is largely preserved. Status codes can be checked in the browser's developer tools under the “Network” tab, with the command-line tool curl, or with crawlers that run through entire websites and report anomalies.

Common mistakes and limits

Correct status codes ensure clean indexing and meaningful error reports. In APIs, they are the basis of error handling: an application can tell from the code whether it should retry, whether authentication is missing, or whether the request was faulty. Frequent mistakes in operation:

• Error page returns status 200 (soft 404)
• 302 instead of 301 for a permanent move
• Redirect chains across several hops
• All deleted pages redirected to the home page across the board
• Maintenance page with status 200 instead of 503

Imple­menta­tion

Nano Concepts records status codes in a redirect list during technical SEO audits and relaunches, sets up redirects and error pages correctly, and monitors the results after launch, so problems surface before visitors or search engines notice them. The results are documented clearly.