Link Building
Referring domain
A referring domain is the normalized host or registrable-domain key used to group one or more source URLs that link to a target.
What it means
Tools may count exact hosts, collapse subdomains to a registrable domain or apply special platform rules. Reproducible counts therefore require URL parsing, case and international-domain normalization, redirect policy, Public Suffix List version and an explicit decision for hosted subdomains.
Why it matters
The contract prevents `www`, protocol variants, country-code suffixes, redirects and multi-tenant hosts from inflating or collapsing the count unpredictably.
Example
Three source URLs on `news.example.co.uk` become one exact-host group; a registrable-domain report may group them under `example.co.uk`, while an unrelated tenant on a public hosting suffix remains separate under the chosen PSL rule.
Common mistakes
Do not strip to the last two labels, mix exact-host and registrable-domain counts, count URL variants as domains, ignore internationalized names, collapse independent hosted tenants or compare tools without their rules.
How AYSA handles this
Signals reviewed
source URL, parsed host, redirect, public suffix, registrable domain, subdomain rule, dataset, snapshot
Problem AYSA can identify
AYSA can identify invalid URLs, duplicate host variants, inconsistent subdomain grouping and counts produced by incompatible rules.
Recommendation prepared
The report publishes both the normalization contract and row-level source mapping before comparisons are made.
Approval preview
The reviewer sees exact-host and grouped-domain values, PSL version, exceptions and the impact of every normalization choice.
Execution
AYSA can apply an approved deterministic grouping rule to imported evidence; it cannot make different crawler datasets exhaustive or identical.
Verification
Test cases cover country-code suffixes, hosted platforms, redirects, internationalized hosts and duplicate URL variants.
Limits
The count depends on crawler coverage and policy choices, and one referring domain can contain many unrelated pages or authors.
Sources and further reading
- WHATWG — URL Living Standard — Web standard
- Public Suffix List — List format and specification — Domain normalization reference
- Semrush — Referring Domains report — First-party vendor documentation
- Semrush — Backlinks report fields — First-party vendor documentation
Written by Marius Dosinescu. Reviewed by AYSA SEO Editorial Team · 2026-07-28 00:00:00.
Quick answers
Frequently asked questions
Is a referring domain always the last two labels of a hostname?
No. Public suffixes such as `co.uk` and hosted platforms require a maintained suffix rule and an explicit grouping policy.
Do all backlink tools count referring domains identically?
No. Coverage, redirect handling, subdomain grouping and Public Suffix List rules can differ.
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