Summary
- Registrar
- REG-GRANSY
- Status
- Created on
- August 22, 2009
- Expires on
- August 22, 2027
- Name servers
- ns2.fastcom.cz, ns6.fastcom.cz, ns1.fastcom.cz, ns5.fastcom.cz
- Mail servers
- mail.fastcom.cz
- DMARC
- Reject
- SPF
- Strict
- DKIM
- 2 keys found
- Website
- See the website
Spoofing protection (DMARC)
E-mails spoofing this domain are rejected.
DMARC tells mail servers what to do with an e-mail that claims to come from fastcom.cz but fails SPF and DKIM checks.
- Policy
- Reject
- Subdomain policy
- Reject
- Messages covered
- 100%
- DKIM alignment
- Relaxed
- SPF alignment
- Relaxed
- Aggregate reports
- [email protected]
- Failure reports
- —
Record
v=DMARC1; p=reject; rua=mailto:[email protected]
Authorized senders (SPF)
E-mails sent by any other server are rejected.
SPF lists the servers allowed to send e-mails for fastcom.cz.
| Rule | Value | Result |
|---|---|---|
| a | — | Allowed |
| mx | — | Allowed |
| ip4 | 91.219.45.239 | Allowed |
| include | spf.protection.outlook.com | Allowed |
| a | admin.fastcom.cz | Allowed |
| include | spf1.supportbox.cz | Allowed |
| a | cp1.fastcom.cz | Allowed |
| all | — | Rejected |
Top-level DNS lookups: 6 of 10 allowed
Record
v=spf1 a mx ip4:91.219.45.239 include:spf.protection.outlook.com a:admin.fastcom.cz include:spf1.supportbox.cz a:cp1.fastcom.cz -all
E-mail signing (DKIM)
DKIM keys published by fastcom.cz under the most common selectors. Other keys may exist under selectors specific to each sending service.
| Selector | Likely service | Key | Status |
|---|---|---|---|
| selector1 | Microsoft 365 | RSA 1024 bits · short | Active |
| default | — | RSA | Active |
RSA keys shorter than 2048 bits, called short keys, are discouraged: they are easier to break.
MX, SPF, DKIM and DMARC: the e-mail records
Four DNS records make a domain's e-mail work. MX says where e-mails sent to fastcom.cz arrive; SPF, DKIM and DMARC let mail servers check that an e-mail claiming to come from fastcom.cz was really sent by this domain, which protects it against spoofing.
MX
An MX (Mail Exchanger) record designates a server that receives e-mails sent to the domain. When someone writes to an @fastcom.cz address, their mail server queries the DNS for the domain's MX records, then delivers the message to the server they point to.
Example record
fastcom.cz MX 10 mx1.fastcom.cz. fastcom.cz MX 20 mx2.fastcom.cz.
What the record contains
- 10, 20
- Priority: the server with the lowest number is tried first. Servers with equal priority share the load.
- mx1.fastcom.cz.
- Host name of the mail server. It must point to an A or AAAA record: an IP address or an alias (CNAME) is not allowed here.
- 0 .
- An MX "0 .", called a null MX, declares that the domain accepts no e-mail: sending servers give up at once instead of retrying.
Declaring at least two servers with different priorities avoids losing e-mails during an outage: sending servers retry for several days and fall back on the backup server. Without any MX, they try to deliver directly to the IP address of the domain.
SPF
SPF (Sender Policy Framework) is a TXT record listing the servers allowed to send e-mails on behalf of the domain. The receiving server checks that the sender is on this list. The final "all" rule says what to do with other servers: reject them (-all), flag their e-mails as suspicious (~all) or make no decision (?all).
Example record
fastcom.cz TXT "v=spf1 mx include:_spf.google.com ip4:192.0.2.10 -all"
What the record contains
- v=spf1
- Identifies the record as an SPF policy. It must come first.
- ip4, ip6
- Allows a specific IP address or an address range, for example ip4:192.0.2.0/24.
- a, mx
- Allows the addresses of the domain itself (A record) or those of its mail servers (MX).
- include
- Allows the servers of another domain, usually a sending service: include:_spf.google.com for Google Workspace.
- -all, ~all
- Final rule, applied to every other server. Its qualifier decides what happens to them: - rejects them, ~ flags their e-mails as suspicious, ? makes no decision, + accepts them (avoid it).
A domain must publish only one SPF record, and its evaluation cannot exceed 10 DNS lookups (include, a, mx, exists, redirect). Beyond that, mail servers treat it as invalid.
DKIM
DKIM (DomainKeys Identified Mail) signs every outgoing e-mail with a private key. The matching public key is published in a TXT record at selector._domainkey.fastcom.cz, so the recipient can check that the message really comes from the domain and was not altered on the way. The selector is chosen by each sending service and cannot be guessed: Dataweaz checks the most common ones (Google Workspace, Microsoft 365, Mailchimp, SendGrid…), so a key published under another selector may exist without appearing here.
Example record
google._domainkey.fastcom.cz TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA…"
What the record contains
- v=DKIM1
- Protocol version. Optional but recommended, in which case it must come first.
- k
- Key algorithm: rsa, the most common, or ed25519.
- p
- Public key, base64-encoded. An empty value means the key has been revoked.
- t
- Flags: y marks a key in testing, whose failures must not be penalized; s forbids its use for subdomains.
Each sending service publishes its key under its own selector (google, selector1, k1…), which allows several keys and seamless rotation. A 2048-bit RSA key is recommended today.
DMARC
DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM. Published on _dmarc.fastcom.cz, it tells mail servers what to do with an e-mail that fails these checks (let it through, send it to spam or reject it) and where to send reports.
Example record
_dmarc.fastcom.cz TXT "v=DMARC1; p=quarantine; rua=mailto:[email protected]; adkim=r; aspf=r"
What the record contains
- v=DMARC1
- Identifies the record as a DMARC policy. It must come first.
- p
- Policy applied to e-mails that fail the checks: none (monitoring only), quarantine (spam folder) or reject.
- sp
- Subdomain policy. Without it, subdomains follow the domain's policy.
- rua
- Address that receives daily aggregate reports about e-mails sent on behalf of the domain, whether they pass the checks or not.
- ruf
- Address that receives detailed failure reports, message by message. Few providers still send them.
- pct
- Percentage of e-mails subject to the policy, for a gradual rollout. 100 by default.
- adkim, aspf
- DKIM and SPF alignment: r (relaxed, a subdomain of the same domain is enough) or s (strict, the domain must be identical).
To roll out DMARC without blocking legitimate e-mails, start with p=none and an rua address, analyze the reports to find every service sending on behalf of the domain, then move to quarantine and finally to reject.
DNS records of fastcom.cz
IP addresses (A and AAAA)
An A record maps the domain name to an IPv4 address: the address of the server that browsers contact to display the website. The AAAA record does the same for an IPv6 address. A domain can have several of them to spread visits across several servers.
| Type | IP address |
|---|---|
| A | 91.219.45.250 |
Name servers (NS)
NS records designate the authoritative DNS servers of the domain: they answer every question about its other records (A, MX, TXT…). They are provided by the registrar, the hosting provider or a dedicated DNS service, and several servers are declared so that the domain keeps working if one of them fails.
| Server | IP address |
|---|---|
| ns2.fastcom.cz | 91.219.45.241 |
| ns6.fastcom.cz | 91.219.45.244 |
| ns1.fastcom.cz | 91.219.45.251 |
| ns5.fastcom.cz | 91.219.45.245 |
Mail servers (MX)
MX records point to the servers that receive e-mails sent to the domain, that is to @fastcom.cz addresses. Each server has a priority: the one with the lowest number is tried first, the others take over if it fails. Without an MX record, the domain cannot receive e-mails.
| Priority | Server | IP address |
|---|---|---|
| 10 | mail.fastcom.cz | 91.219.45.239 |
TXT records
TXT records hold free text. They are used to prove ownership of the domain to online services (Google, Microsoft, Facebook…) and to publish e-mail security rules: SPF on the domain itself, DMARC on _dmarc.fastcom.cz.
| Name | Content |
|---|---|
| fastcom.cz | v=spf1 a mx ip4:91.219.45.239 include:spf.protection.outlook.com a:admin.fastcom.cz include:spf1.supportbox.cz a:cp1.fastcom.cz -all |
| _dmarc.fastcom.cz | v=DMARC1; p=reject; rua=mailto:[email protected] |
| selector1._domainkey.fastcom.cz | v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDB4jzQ1vFT71rbLtOdu4ynz30LV85jHlrR5bFAaegHCwZgBiyPDGj+CrwniPN4HhAwsaroFE7VwSLNdCUupSeAgUTXlw6BOO4fTi7DQxCeTuaE6r5M/t4NjnnOFqlIzWQULcWnkTW5g7DqNfiOwqh0bRCo2XUmFlolDM6vnZoSsQIDAQAB; n=1024,1455848523,1471573323 |
| default._domainkey.fastcom.cz | v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzBeGxkygf6UlZ55Rk5zWu79wHYXY3KlIS7uNYNMv50d5zNGNI/VlF0eyKPHZGnkneHC5iyrDlWELkxRhue0MpO9cXPTfbqI8xaMF2W0uGa3liKl/NErZJP0W3qPoeTdaJBS4oOltQ08Bk9pP2CVV/wLcRqn0fERPGlz9wDJnohLCMeMOd9G7eDgBsd1ymQNvQVjBPSBiMawnv9w7hZdvWRv6BeF8IAfEt5KzVpRcNN2qIAL176T0GetB3H0OtuRPiuSM6tuDmCIVnCw5xb73G1pKnw1SCchlB7 7zHBu3w84 Hu4ezHWrSLnJfm0 anrX5gDocQRpn vq/fW8chfTwIDAQAB; |
Related IP addresses
IPv4 addresses used by fastcom.cz: website, name servers, mail servers and senders authorized by SPF.
| IP address | Used for | /24 block |
|---|---|---|
| 91.219.45.250 | Website | 91.219.45.0/24 |
| 91.219.45.241 | Name server ns2.fastcom.cz | 91.219.45.0/24 |
| 91.219.45.244 | Name server ns6.fastcom.cz | 91.219.45.0/24 |
| 91.219.45.251 | Name server ns1.fastcom.cz | 91.219.45.0/24 |
| 91.219.45.245 | Name server ns5.fastcom.cz | 91.219.45.0/24 |
| 91.219.45.239 | Mail server mail.fastcom.cz, Authorized by SPF | 91.219.45.0/24 |
WHOIS of the domain fastcom.cz
% (c) 2006-2021 CZ.NIC, z.s.p.o. % % Intended use of supplied data and information % % Data contained in the domain name register, as well as information % supplied through public information services of CZ.NIC association, % are appointed only for purposes connected with Internet network % administration and operation, or for the purpose of legal or other % similar proceedings, in process as regards a matter connected % particularly with holding and using a concrete domain name. % % Full text available at: % http://www.nic.cz/page/306/intended-use-of-supplied-data-and-information/ % % See also a search service at http://www.nic.cz/whois/ % % % Whoisd Server Version: 3.15.0 % Timestamp: Thu Oct 01 12:15:22 2026 domain: fastcom.cz registrant: SUB-1693763 admin-c: SUB-1693763 nsset: SUB-20260529214502 registrar: REG-GRANSY registered: 22.08.2009 04:41:31 changed: 29.05.2026 21:45:03 expire: 22.08.2027 contact: SUB-1693763 org: Fast Communication s.r.o. name: Pavel Valousek address: Ke Klubovne 1650/9 address: Praha 5 address: 155 00 address: CZ address: CZ registrar: REG-GRANSY created: 24.05.2026 21:10:02 changed: 01.06.2026 10:02:19 nsset: SUB-20260529214502 nserver: ns1.fastcom.cz (91.219.45.251) nserver: ns2.fastcom.cz (91.219.45.241) nserver: ns5.fastcom.cz (91.219.45.245) nserver: ns6.fastcom.cz (91.219.45.244) tech-c: SUB-1693763 registrar: REG-GRANSY created: 29.05.2026 21:45:02 % Query time: 341 msec % WHEN: Thu Oct 01 10:15:22 UTC 2026